Clear the copy of the offscreen texture used by shared contexts after the DEPTH, STENCIL

attachments have been resized since FBOs don't work if the attachment texture sizes don't match.
BUG=47185
Review URL: http://codereview.chromium.org/2872010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@50499 0039d316-1c4b-4281-b951-d872f2087c98