Drop unecessary use of CrossThreadPersistent by CanvasAsyncBlobCreator.

The callback object that the CanvasAsyncBlobCreator passes along to a
background thread can be kept as a simple Member<>; no need to
involve CrossThreadPersistent<> and risk inadvertently introducing
leaks.

R=
BUG=

Review-Url: https://codereview.chromium.org/2051993002
Cr-Commit-Position: refs/heads/master@{#399181}
2 files changed