[Ozone] Removing Stub SFO implementation & CreateTestHelper from SFO

[Ozone] Removing Stub SFO implementation & CreateTestHelper from SFO

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=

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=245377

[dnicoara, 2014-01-16 16:23:07]

PTAL

[rjkroege, 2014-01-16 17:46:15]

lgtm

[sadrul, 2014-01-16 17:47:11]

lgtm

[commit-bot: I haz the power, 2014-01-16 19:36:58]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/dnicoara@chromium.org/131493005/1

[commit-bot: I haz the power, 2014-01-16 21:36:24]

Retried try job too often on win_rel for step(s) unit_tests
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=win_rel&nu...

[commit-bot: I haz the power, 2014-01-16 22:06:08]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/dnicoara@chromium.org/131493005/1

[commit-bot: I haz the power, 2014-01-16 23:44:11]

Change committed as 245377