Have Document::getCSSCanvasElement() return a reference

Have Document::getCSSCanvasElement() return a reference instead of a pointer as
it can never return null. It makes the calling code look safer and gets rid of
some unnecessary null checks.

R=adamk

Review URL: https://codereview.chromium.org/181823002

git-svn-id: svn://svn.chromium.org/blink/trunk@167934 bbb929c8-8fbe-4397-9dbb-9b2b20218538