Missing icon assets in golden tests

Filing this to investigate why some of the assets we use as icon images are missing during macos golden testing.

The changes in https://github.com/flutter/devtools/pull/9304/files caused some icons that weren't being rendered previously in our macos golden tests to appear, as well as others that were being rendered in our macos golden tests to disappear.

The assets that were previously missing are:
* `packages/devtools_app/icons/material_symbols/step_over.png`
* `packages/devtools_app/icons/material_symbols/step_into.png`
* `packages/devtools_app/icons/material_symbols/step_out.png`

The asset that is now missing is:

* `packages/devtools_app/icons/flutter.png`

All 4 of the assets display as expected in a running app, and all 4 are included in the pubspec:
* https://github.com/flutter/devtools/blob/8e6559236a6464104e05ced03be1a2ca80e7f725/packages/devtools_app/pubspec.yaml#L96
* https://github.com/flutter/devtools/blob/8e6559236a6464104e05ced03be1a2ca80e7f725/packages/devtools_app/pubspec.yaml#L100

---

### `packages/devtools_app/test/test_infra/goldens/codeview_scrollbars.png`:

Note: The code for these buttons was not touched in https://github.com/flutter/devtools/pull/9304: https://github.com/flutter/devtools/blob/a62c33884a355eef0bd351c8e0d5778d393c0b0b/packages/devtools_app/lib/src/screens/debugger/controls.dart#L121-L145

### `packages/devtools_app/test/test_infra/goldens/logging/metadata_chips.png`

Note: The code for these icons also didn't change, except for the icon size becoming `const`:

Start by reviewing packages/devtools_app/pubspec.yaml and the icon references in packages/devtools_app/lib/src/screens/debugger/controls.dart, then inspect the affected golden files under packages/devtools_app/test/test_infra/goldens. Run the macOS golden tests and compare rendering of the four listed assets; done means the expected icons appear consistently in the affected goldens.

Written by the indexing model from the issue text.