[TaskScheduler] Migrate rest of page_capture_api.cc

Put ShareableFileReference's storage::ScopedFile on correct task runner
from FILE thread. FILE thread is deprecated. The correct task runner
here requires blocking IO and also requires TaskShutdownBehavior to
BLOCKING_SHUTDOWN.

BLOCKING_SHUTDOWN is necessary because ShareableFileReference
creates a storage::ScopedFile object inside it whose responsibility is
to delete file through
~storage::ScopedFile -> storage::ScopedFile::Reset ->
  (PostTask) base::DeleteFile.

BLOCKING_SHUTDOWN will ensure that DeleteFile is always called
when necessary.

Bug: 689520
Change-Id: Ib1943fb20fbd014b8ecfd32877323d88e0fe6761
Reviewed-on: https://chromium-review.googlesource.com/604932
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Istiaque Ahmed <lazyboy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#492519}
2 files changed