Stabilize Storage tests

Make it so that upload mock waits not on setting up expectations (that
was a mistake), but on actual completion of the upload. As a side-effect
it was now possible to remove no longer needed "Possible(...)"
expectations in some places.
To simplify the tests, added TestCallbackAutoWaiter RAII wrapper to
test_support_callback and used it in the tests.

Bug: 1194245
Bug: 1194076
Change-Id: Iacc603b620fec2ddefc17d408f1658300960d087
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2798041
Reviewed-by: Zach Trudo <zatrudo@google.com>
Commit-Queue: Leonid Baraz <lbaraz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#868282}