Adds support for capturing a sub rect of the compositing surface.

Also removes the old PopulateBitmapWithContents API.

BUG=345865

Review URL: https://codereview.chromium.org/173023005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253898 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253898