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As part of onion souping domstorage this adds a couple of new classes
to blink more or less duplicating the content equivalents:

CachedStorageArea matches content::LocalStorageCachedArea (with some
minor API changes, in particular around how it acquires a mojom
StorageArea). Also has improved test coverage compared to the content
version.

MockStorageArea is very similar to content::MockLevelDBWrapper (with
the difference that it doesn't implement StoragePartitionService since
it doesn't need to with the changed API of CachedStorageArea).

Bug: 781870
Change-Id: Ic64b3bf307fe275d9287669743f186c8470847bb
Reviewed-on: https://chromium-review.googlesource.com/1166218Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Commit-Queue: Daniel Murphy <dmurph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582334}