[dom_distiller] Fix two issues with distiller.

1) On pages with no images, ensure that the callback is still called.
2) Fix crash after running the image fetch. Since we were deleting the
DistillerURLFetcher which owned the |id| it can be junk before erasing
from |image_fetchers_|
BUG=321725

Review URL: https://codereview.chromium.org/80213002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236650 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236650