Introduce common gmock matchers for USS

We adopt them already in some existing tests and plan to use them in
future patches as part of the migration of session sync to USS.

Bug: 681921
Change-Id: I91bcb5b2d0383be89bef1b905e1d3b16259f94bb
Reviewed-on: https://chromium-review.googlesource.com/983918
Reviewed-by: Jan Krcal <[email protected]>
Commit-Queue: Mikel Astiz <[email protected]>
Cr-Commit-Position: refs/heads/master@{#549525}
9 files changed