Makes ImageView do a bit more checking before assuming images equal

This covers the case of the storage not changing, but the bits in the
storage changing, as happens with extensions.

BUG=319251
TEST=see bug.
R=pkotwicz@chromium.org

Review URL: https://codereview.chromium.org/73893007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235691 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235691