MockClock for ScopedTaskEnvironment

Add the option to get a MockClock that is in sync with the underlying
MockTickClock that ScopedTaskEnvironment uses. Some test need access to
a base::Clock and ScopedTaskEnvironment currently just provides a
MockTickClock.

Need for this araised while refactoring
components/ntp_snippets/breaking_news/breaking_news_gcm_app_handler_unittest.cc
to use ScopedTaskEnvironment instead of MessageLoop.

Bug: 891670
Change-Id: I0716dbe2b46e268dfe57bed7c667d275f62519af
Reviewed-on: https://chromium-review.googlesource.com/c/1356553
Commit-Queue: Carlos Caballero <carlscab@google.com>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614342}
3 files changed