Define a --force-gpu-rasterization flag

This flag forces gpu rasterization for all layers (that is, it behaves the
way --enable-gpu-rasterization behaved previously).

--enable-gpu-rasterization now enables gpu rasterization only on layers
that have a GPU rasterization hint. Since this hint will be added in a
later CL, this flag is currently a no-op.

BUG=329722

Review URL: https://codereview.chromium.org/189883009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256143 0039d316-1c4b-4281-b951-d872f2087c98
chromeos: Document external display brightness histograms.

Add documentation for the following metrics:

  Power.ExternalBrightnessReadResult
  Power.ExternalBrightnessRequestResult
  Power.ExternalBrightnessWriteResult
  Power.ExternalDisplayOpenResult

BUG=chromium:315371

Review URL: https://codereview.chromium.org/192563002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256144 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256144