canvas2d: Always check for lost context.

R=fserb@chromium.org

Bug: 976136
Change-Id: If2bd977fca8c7fecff485c7735abc5c487f8666a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1670290
Auto-Submit: Khushal <khushalsagar@chromium.org>
Commit-Queue: Fernando Serboncini <fserb@chromium.org>
Reviewed-by: Fernando Serboncini <fserb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#671442}