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In order to support moving CacheStorage off the IO thread and on to
a separate SequencedTaskRunner, we are encapsulating the threading
requirements.  This CL does this by removing the requirement that
external clients of CacheStorageContext must be on the IO thread.
Instead the API can be accessed directly on the UI thread and
CacheStorageContextImpl will handle executing the code on the correct
sequence.

Bug: 960012
Change-Id: I7d576a7deb484113cff0e9454b038f7b87c5e503
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1625971Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Christian Dullweber <dullweber@chromium.org>
Commit-Queue: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662961}