Fix integer overflow in software compositor

      Ensure that the size mapped from the renderer
process for the software frame is not less than
expected due to integer overflow.

BUG=348332

Review URL: https://codereview.chromium.org/196283018

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@257417 0039d316-1c4b-4281-b951-d872f2087c98