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1) Make StorageModule to be instantiated only by factory method
2) Make StorageModule overridable by TestStorageModule
3) Allow TestStorageModule to be constructed directly

In the process I started to adjust tests, and ended up with rather
significant refactoring there too:
4) Use TestEvent instead of callback_ and result_ in the test class;
   it allows to make multiple calls in the same test fixture (not
   happening right now, but will likely be needed in the future).
5) Eliminate specialized test classes and use MOCK_METHOD + ON_CALL +
   optionally WillByDefault to achieve the same result. This makes sure
   that later changes in one test class are not skipped in another -
   we now have only one.

Bug: b:153364303
Change-Id: I4f7bcc956380c8b313ca4c2dc1f5ee6c716f586c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2277505Reviewed-by: Zach Trudo <zatrudo@google.com>
Commit-Queue: Leonid Baraz <lbaraz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#784494}