Update RenderImage when the image src is cleared

When the src is removed via JavaScript, the image loader now
updates the renderer.

BUG=123017

Review URL: https://chromiumcodereview.appspot.com/20473002

git-svn-id: svn://svn.chromium.org/blink/trunk@155294 bbb929c8-8fbe-4397-9dbb-9b2b20218538

Task-number: QTBUG-38287
Change-Id: I49c353cfa4ad1f7c1ceac80df81b8cb713d291df
Reviewed-by: Michael Bruning <michael.bruning@digia.com>