Restrict the File System Provier API permission to Chrome OS only.

This API implementation is compiled on Chrome OS only, so the permission should
be available on Chrome OS only.

TEST=Tested manually that still works on Chrome OS.
BUG=248427

Review URL: https://codereview.chromium.org/581303002

Cr-Commit-Position: refs/heads/master@{#295918}
2 files changed