Allow empty overhang image

When resources fail to load (e.g. tests), the overscroll theme images are not
NULL, they're empty, so handle accordingly.
This is needed to unblock https://codereview.chromium.org/236363019/

BUG=362164

Review URL: https://codereview.chromium.org/246633002

git-svn-id: svn://svn.chromium.org/blink/trunk@172287 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Switch some functions in RenderObject to reference

These call sites were always called on a non-null renderer
(most of them |this|) so changed them to take a reference.

Review URL: https://codereview.chromium.org/246683005

git-svn-id: svn://svn.chromium.org/blink/trunk@172288 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Adjust Screen's derived Oilpan type after r172152

Following r172152, Screen is no longer an EventTarget. Hence we can
simplify it to derive from GarbageCollectedFinalized only.

R=
BUG=162827

Review URL: https://codereview.chromium.org/246253008

git-svn-id: svn://svn.chromium.org/blink/trunk@172289 bbb929c8-8fbe-4397-9dbb-9b2b20218538