(Partially) remove threading from RendererGpuVideoAcceleratorFactories

In most cases, the entry points to RendererGpuVideoAcceleratorFactories
are called from its factory loop, so trampolining is unnecessary.

This change should be a functional no-op according to the current
architecture.

BUG=306333
TEST=local build, run on CrOS snow, build on desktop Linux

Review URL: https://codereview.chromium.org/27223007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229693 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229693