Drop |background_task_runner| parameter from async snapshot.h methods.

TaskRunners being passed in were already all parallel anyways, so use
base::PostTask directly.

Intentionally not setting TaskPriority in TaskTraits as the default is
to inherit priority from the caller's context.

Cleans up one more GetBlockingPool() callsite.

Bug: 667892
Change-Id: Ic1a919945496a6bc49579ef684b810112d7960bf
Reviewed-on: https://chromium-review.googlesource.com/600531
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#492506}
24 files changed