Unreviewed gardening: Rebaseline scrollbar image diffs post r156352

TBR=bokan@chromium.org
BUG=249425

Review URL: https://codereview.chromium.org/23140010

git-svn-id: svn://svn.chromium.org/blink/trunk@156361 bbb929c8-8fbe-4397-9dbb-9b2b20218538
AX: Check renderer type before casting.

We assumed some types of elements have specific types of renderers.
e.g. <img>'s renderer is always a RenderImage.

This assumption is going to be broken when we support author shadow DOM.
So we should check renderer types explicitly.

BUG=253801,274205
TEST=none; No behavior changes yet.

Review URL: https://chromiumcodereview.appspot.com/23224012

git-svn-id: svn://svn.chromium.org/blink/trunk@156362 bbb929c8-8fbe-4397-9dbb-9b2b20218538