Remove unnecessary CanvasAsyncBlobCreator keep alive protection.

Closures keep their GCed arguments and |this| alive until completed,
hence manual keep-alive handling on top of that isn't required
for CanvasAsyncBlobCreator.

R=haraken
BUG=

Review URL: https://codereview.chromium.org/1882563003

Cr-Commit-Position: refs/heads/master@{#387260}
2 files changed