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This CL makes the following changes:
1) Change to run image folder cleanup after Chrome startup using a background
   task, rather than running it at Chrome shutdown using a high priority task.
2) Retain the image folder to avoid deleting and creating it repeatedly.
3) Remove <subdir> from the full path for the temp file, which is unnecessary.
4) Delete the temp files in batch.

These changes have the following benefits:
1) Fix the issue of Chrome shutdown being blocked by image file IO for some users
   as in the current implementation.
2) Save some disk IO related to the sub-directories in the image folder and the
   image folder itself.
3) Avoid creating a deletion task for each file, otherwise the overhead can be
   large when there is a steady stream of notifications coming rapidly.

Bug: 888276
Change-Id: I214680aa00bd7f19e84e207e82ada553583a094b
Reviewed-on: https://chromium-review.googlesource.com/c/1260498
Commit-Queue: Xi Cheng <chengx@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Greg Thompson <grt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601227}