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This insulates LocalStorageContextMojo behind a new
LocalStorageControl mojom interface defined by the Storage Service.

DOMStorageContextWrapper no longer has direct access to the
LocalStorageContextMojo instances, instead interacting with them only
through the mojom interface.

This simplifies DOMStorageContextWrapper, but also helps prepare
LocalStorageContextMojo for migration into private Storage Service
code.

Bug: 1000959
Change-Id: I12c6ece0b1dd01c3e17e66e6249b2d01935675f6
Tbr: jam@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1902244Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Commit-Queue: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/master@{#717745}