Implement WebKit::Platform::isThreadedCompositingEnabled()

Implement WebKit::Platform::isThreadedCompositingEnabled()

This provides implementations of WK::Platform::isThreadedCompositingEnabled()
for content/renderer/renderer_webkitplatformsupport_impl (aka the one we really
use) and for the unit / layout test support logic in webkit/. The real
implementation depends on content::RenderThreadImpl::compositor_thread(), the
test implementations have to be set explicitly by the test harness. Neither
depend on WebCompositorSupport, although that's still implemented so everything
still passes before the WebKit side at https://bugs.webkit.org/show_bug.cgi?id=111386
lands and rolls in.

R=enne@chromium.org


Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=186111

[jamesr, 2013-03-05 01:49:22]

This is the first step towards relanding
https://codereview.chromium.org/12386078. Plan is land this, then roll into
WebKit to land https://bugs.webkit.org/show_bug.cgi?id=111386, then roll that
back into Chromium and land https://codereview.chromium.org/12386078 + delete
the WebCompositorSupport stuff.

[enne (OOO), 2013-03-05 02:04:14]

lgtm

[commit-bot: I haz the power, 2013-03-05 02:08:24]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/jamesr@chromium.org/12378094/1

[commit-bot: I haz the power, 2013-03-05 04:22:15]

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

[commit-bot: I haz the power, 2013-03-05 05:01:51]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/jamesr@chromium.org/12378094/1

[commit-bot: I haz the power, 2013-03-05 06:03:50]

Change committed as 186111