--sandbox_add_mount_pair ignored on OSX

Please provide the following information. The more we know about your system and use case, the more easily and likely we can help.

### Description of the problem / feature request / question:

I would like to use `--sandbox_add_mount_pair` to mount specific directories into the sandbox filesystem on both OSX and Linux, but [this support was only added on Linux](https://github.com/bazelbuild/bazel/commit/18e6b410a7f4e24b67740978d101cd665edea6cc).

### If possible, provide a minimal example to reproduce the problem:

[DarwinSandboxedSpawnRunner](https://github.com/bazelbuild/bazel/blob/04f2b03d351d3de16601dff2290f40643f2cb16e/src/main/java/com/google/devtools/build/lib/sandbox/DarwinSandboxedSpawnRunner.java) is missing `sandboxAdditionalMounts`, but [LinuxSandboxedSpawnRunner](https://github.com/bazelbuild/bazel/blob/04f2b03d351d3de16601dff2290f40643f2cb16e/src/main/java/com/google/devtools/build/lib/sandbox/LinuxSandboxedSpawnRunner.java) has them.

### Environment info
* Operating System:

macOS Sierra 10.12.6

* Bazel version (output of `bazel info release`):

release 0.7.0-homebrew

Read DarwinSandboxedSpawnRunner.java and compare its handling with LinuxSandboxedSpawnRunner.java, focusing on the missing sandboxAdditionalMounts support. Run a macOS reproduction using --sandbox_add_mount_pair and verify that the requested directories are mounted into the sandbox filesystem, matching the stated Linux behavior.

Written by the indexing model from the issue text.