Implemented switch to override the size of the GPU command buffer

BUG=315429
R=kbr@chromium.org, piman@chromium.org

Review URL: https://codereview.chromium.org/61343002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233643 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233643