Add flag to disable repaint after layout.

When repaint-after-layout is enabled by default this flag will allow us to
turn the feature off at runtime if needed.

BUG=320139

Review URL: https://codereview.chromium.org/145273026

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248716 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248716