Deprecate FileSystemMountPointProvider::GetFileSystemRootPathOnFileThread

This was initially introduced to get ExternalFileSystem's
local mount root path, but accessing MountPointProvider
from lower-layer FileUtil module looks ugly.

This patch:
- Deprecate FileSystemMountPointProvider::GetFileSystemRootPathOnFileThread
- Add FileSystemOperationContext::root_path() instead for passing root path

BUG=241701
TEST=Moved tests from content_unittests:FileSystemMountPointProviderTest.*
 to content_unittests:SandboxMountPointProviderTest.*

[email protected], [email protected], [email protected]

Review URL: https://codereview.chromium.org/15959006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@202776 0039d316-1c4b-4281-b951-d872f2087c98
20 files changed