--enable-gpu-rasterization flag for turning on hybrid ganesh mode

BUG=

Review URL: https://codereview.chromium.org/85813003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237482 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237482