The platform_canvas function is not usable when PaintCanvas is a
different type than SkCanvas.  Route callers through a new function
instead.

There a number of callers that still want to use SkCanvas, so
have left the original function.

Since all of these GetWritablePixels callers are using
SkCanvasVideoRenderer, take the opportunity to properly convert
that to PaintCanvas as well.

BUG=671433
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel

Review-Url: https://codereview.chromium.org/2686173003
Cr-Commit-Position: refs/heads/master@{#449752}