Revert "Revert "Enable composited iframes on hi-DPI.""

This was a speculative revert. It turned out not to be the problem. Relanding.

This reverts commit 9797adb04362cd267e9727c506deebade23446d8.

TBR=zeeshan@chromium.org
BUG=306566

Review URL: https://codereview.chromium.org/27709003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229126 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229126