Revert "Enable composited iframes on hi-DPI."

This reverts commit 6dd2053b0e326cb7f221d6f7820bd444d7257df1.

TBR=zeeshan@chromium.org
BUG=306566

Review URL: https://codereview.chromium.org/26973005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228420 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228420