Dirty rects always contain full tiles with delegated rendering.

Fix: Compute update_rect separately from original dirty rects and pass
them to update_rect_ as well as updating the contents.

BUG=316469

Review URL: https://codereview.chromium.org/85143002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238911 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238911