Update Oilpan related comment in GraphicsContext destructor.

I was too fast in concluding that these asserts are not interesting.
There is exactly one case where they do not hold. We should reenable
these asserts for the Oilpan build (but find a different way of
allowing CanvasRenderingContext2D to violate them). For now, updated
comment and filed a bug report to track this.

R=junov@chromium.org
BUG=370793

Review URL: https://codereview.chromium.org/269283006

git-svn-id: svn://svn.chromium.org/blink/trunk@173551 bbb929c8-8fbe-4397-9dbb-9b2b20218538