ui:: Add --ui-enable-impl-side-painting flag.

When present, the ui::Compositor enables impl-side painting,
and uses cc::PictureLayer instead of cc::ContentLayer.

BUG=350737

Review URL: https://codereview.chromium.org/189543007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256850 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256850