Serialize <input type="image"> images

Add support for serializing images specified in an input tag with
type image. This images is extracted in the PageSerializer and added
as a SerializedResource.

R=
BUG=248867

Review URL: https://chromiumcodereview.appspot.com/16520007

git-svn-id: svn://svn.chromium.org/blink/trunk@155598 bbb929c8-8fbe-4397-9dbb-9b2b20218538