Clean up mac gpu switching code a little.

Clean up mac gpu switching code a little.

This may not helps with the crash we observed in the bug report, but at
least the code is more testable.

Also, re-enable the gpu_data_manager_impl_private_unittest.cc which was
accidently disabled.

BUG=361991
TEST=content_unittests
R=kbr@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=263971

[Zhenyao Mo, 2014-04-14 21:58:38]

kbr: please take a look.

[Ken Russell (switch to Gerrit), 2014-04-15 00:25:45]

LGTM.

The compilation failures are caused by this patch, so obviously, please fix
before committing.

[Zhenyao Mo, 2014-04-15 00:36:06]

The CQ bit was checked by zmo@chromium.org

[commit-bot: I haz the power, 2014-04-15 00:36:57]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/zmo@chromium.org/237903002/20001

[commit-bot: I haz the power, 2014-04-15 01:11:10]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-04-15 01:11:11]

Try jobs failed on following builders:
  tryserver.chromium on linux_chromium_chromeos_clang_dbg

[Zhenyao Mo, 2014-04-15 16:55:18]

The CQ bit was checked by zmo@chromium.org

[commit-bot: I haz the power, 2014-04-15 16:57:36]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/zmo@chromium.org/237903002/30001

[commit-bot: I haz the power, 2014-04-15 19:41:29]

Change committed as 263971