ImageBitmaps need not hold onto an ImageBuffer object.

This patch address crbug.com/273377 which showed that ImageBuffers weren't
being ref passed between ImageBitmaps. We instead create temporary ImageBuffers.
 
BUG=273377, 166658

Review URL: https://chromiumcodereview.appspot.com/23072025

git-svn-id: svn://svn.chromium.org/blink/trunk@156566 bbb929c8-8fbe-4397-9dbb-9b2b20218538