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In reduced memory mode of Chrome, the browser context will be loaded
asynchronously, so we need to wait until the profile being created and
then retrieve the BlobStorageContextGetter from a Profile.

This CL creates an interface to retrieve the BlobStorageContextGetter
in a callback to download service.

TBR=peter@chromium.org

Bug:959208

Change-Id: I444f9836324428ed285ac67a135e14f6380babc8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1591948Reviewed-by: Xi Han <hanxi@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Reviewed-by: Min Qin <qinmin@chromium.org>
Commit-Queue: Xing Liu <xingliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#657318}