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As part of removing wtf/time.h we are replacing almost all the usages of
WTF::ScopedMockClock by base::'s TestMockTimeTaskRunner. The remaining
migrations, web_view_test.cc and window_performance_test.cc, will be
done in a followup CL as they require additional changes.

The migrations are all pretty similar. A Clock/TickClock object is added to the
class that is being tested. That class will be internally used to retrieve the
current time/timeticks instead of directly using base/time functions. Production
code will use the DefaultClock/DefaultTickClock which are mostly wrappers to
base::Time::Now() and base::TimeTicks::Now() so there is no change in behaviour
(apart from an extra indirection). Using a clock class has the benefit of
allowing tests to overwrite, by using Set{Clock|TickClock}ForTesting(), that
clock with a mock one they can handle with no changes in the production code.

Note that setting a mock clock sometimes requires additional changes like
recomputing some timestamps that were calculated at object's creation time using
the default (real time) clock.

Last but not least, by using TestMockTimeTaskRunner we are avoiding all the
complications that overwritting the global time functions brings, like the lack
of protection of those global pointers or the inherent disparities of using a
mock and a real clock at the same time.

Bug: 919383
Change-Id: I4f1882643ae817cf68a05f2583a9756f4deff554
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1518518
Commit-Queue: Sergio Villar <svillar@igalia.com>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#660347}