Do not elevate image's decodeCachePriority if an ImageBitmap is empty.

R=junov@chromium.org,senorblanco@chromium.org
BUG=166658

Review URL: https://chromiumcodereview.appspot.com/22748002

git-svn-id: svn://svn.chromium.org/blink/trunk@155938 bbb929c8-8fbe-4397-9dbb-9b2b20218538