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