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Given we now use the OzonePlatform to initialize SFO and we already have a test
implementation(the file backend), the Stub implementation isn't required. After
the removal the unittests will default to using the file backend.

This change also fixes the recent brakage of aura_unittests since the Stub SFO
returns 0 (kNullAcceleratedWidget) for a widget, which triggers a DCHECK
failure in WindowTreeHost::DestroyCompositor.

BUG=

Review URL: https://codereview.chromium.org/131493005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245377 0039d316-1c4b-4281-b951-d872f2087c98