Introduce type casting helper functions for CanvasRenderingContext subclasses.

It is helpful to detect bad casts.

BUG=309516
TEST=none; no behavior changes.

Review URL: https://codereview.chromium.org/98553002

git-svn-id: svn://svn.chromium.org/blink/trunk@162970 bbb929c8-8fbe-4397-9dbb-9b2b20218538