[Build] WebGPU EP fails to build for iOS: Dawn's ObjCUtils.mm is incompatible with ORT's forced ObjC ARC

### Describe the issue

Building the WebGPU EP for iOS fails to compile. Dawn's `src/dawn/utils/ObjCUtils.mm` uses manual `retain` / `release`, while ORT's own iOS toolchain file forces ARC:

- `cmake/onnxruntime_ios.toolchain.cmake:11` → `SET(CMAKE_XCODE_ATTRIBUTE_CLANG_ENABLE_OBJC_ARC "YES")`
- `dawn-src/src/dawn/utils/ObjCUtils.mm` → `[static_cast(layer) retain]` / `release`

Dawn guards this file behind a plain `elseif (APPLE)` in `src/dawn/utils/CMakeLists.txt`, which is true for iOS as well as macOS — but the code itself assumes MRC, which only holds for the macOS build.

`ObjCUtils.mm` belongs to the `dawn_system_utils` target, so it is not excluded by `DAWN_BUILD_SAMPLES=OFF` / `DAWN_BUILD_TESTS=OFF` (both of which ORT already sets).

**This is the only file in the build that fails.** Everything else — including the WebGPU EP itself and Dawn's Metal backend — compiles for iOS. So the gap looks narrow.

Errors:

```
_deps/dawn-src/src/dawn/utils/ObjCUtils.mm:36:10: error: cast of C pointer type 'void *' to
Objective-C pointer type 'CALayer *' requires a bridged cast
_deps/dawn-src/src/dawn/utils/ObjCUtils.mm:36:39: error: 'retain' is unavailable:
not available in automatic reference counting mode
_deps/dawn-src/src/dawn/utils/ObjCUtils.mm:36:39: error: ARC forbids explicit message send of 'retain'
_deps/dawn-src/src/dawn/utils/ObjCUtils.mm:42:39: error: ARC forbids explicit message send of 'release'
_deps/dawn-src/src/dawn/utils/ObjCUtils.mm:48:16: error: no matching conversion for functional-style
cast from 'CALayer * _Nonnull' to 'ScopedCALayer'
```

The compile line does contain `-fobjc-arc`, confirming where it comes from.

#### Proposed fix

Disabling ARC for just that one file, after Dawn is made available, in `cmake/external/onnxruntime_external_deps.cmake`:

```cmake
onnxruntime_fetchcontent_makeavailable(dawn)

if (CMAKE_SYSTEM_NAME STREQUAL "iOS")
set_source_files_properties(
"${dawn_SOURCE_DIR}/src/dawn/utils/ObjCUtils.mm"
DIRECTORY "${dawn_SOURCE_DIR}/src/dawn/utils"
PROPERTIES COMPILE_OPTIONS "-fno-objc-arc")
endif()
```

Note that `set_source_files_properties` is directory-scoped, so both the placement *after* `makeavailable` and the explicit `DIRECTORY` argument matter — setting it earlier or without `DIRECTORY` silently has no effect.

An upstream-side alternative would be for Dawn to use `CFRetain` / `CFRelease` plus a `__bridge` cast, which compiles under both ARC and MRC.

> Status: I am reporting the blocker itself, which is fully reproducible. The proposed cmake fix is still being verified by a full rebuild on my side; I will follow up in a comment with the result either way.

### Urgency

Not urgent for us — we are evaluating, not shipping on this yet. Reporting it because #24308 ("Support WebGPU build for android and ios") suggests iOS is supported, and this makes the iOS half not build at all with the released v1.29.0 sources.

### Target platform

iOS

### Build script

```bash
python3 tools/ci_build/build.py \
--build_dir ./build/ios \
--config Release --parallel --skip_tests --skip_submodule_sync \
--ios --apple_sysroot iphoneos --osx_arch arm64 --apple_deploy_target 16.0 \
--use_webgpu --cmake_generator Xcode \
--cmake_extra_defines onnxruntime_BUILD_UNIT_TESTS=OFF
```

(Two notes for anyone reproducing: `--cmake_generator Xcode` is mandatory — `build.py` rejects Ninja for iOS at `build.py:935`, and that rejection message mentions "framework build" even when `--build_apple_framework` is not passed, which is a little misleading. Also, if Homebrew's `onnx` / `protobuf` are installed, `brew unlink onnx protobuf abseil` is needed first, otherwise `/opt/homebrew/include` shadows ORT's vendored onnx and protobuf.)

### Error / output

See above.

### Visual Studio Version

N/A

### GCC / Compiler Version

Apple clang 17.0.0 (clang-1700.6.3.2), Xcode on macOS 26.1, Apple M3 Pro. ONNX Runtime v1.29.0, Dawn v20260714.215939 (as pinned in `cmake/deps.txt`).

Start in cmake/external/onnxruntime_external_deps.cmake and inspect how Dawn is made available, then review dawn-src/src/dawn/utils/ObjCUtils.mm and its target directory. Reproduce with the provided iOS build command and verify the proposed file-specific ARC handling. Done means the WebGPU EP completes an iOS build without the ObjCUtils.mm errors.

Written by the indexing model from the issue text.