Add note in base::ScopedMockClockOverride documentation

This just duplicates documentation from
base::subtle::ScopedTimeClockOverrides but I thought it was worth adding
it here, as ScopedMockClockOverride doesn't currently fix any of the
limitations.

Also, because of this limitation we can't use ScopedMockClockOverride as-is
in Blink tests. We create a task scheduler and a thread pool before running
tests on the main thread (and also call TimeTicks::Now from one of the
threads created).

Bug: 866930
Change-Id: If7b081bd980b8ac8780c892c21b082923edd1a29
Reviewed-on: https://chromium-review.googlesource.com/c/1324103Reviewed-by: kylechar <kylechar@chromium.org>
Commit-Queue: Adithya Srinivasan <adithyas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606502}

base/test/scoped_mock_clock_override.h

@@ -26,6 +26,10 @@ namespace base {
 // For new unit tests, developers are highly encouraged to structure new code
 // around a dependency injected base::Clock, base::TickClock, etc. to be able
 // to supply a mock time in tests without a global override.
+//
+// NOTE: ScopedMockClockOverride should be created while single-threaded and
+// before the first call to Now() to avoid threading issues and inconsistencies
+// in returned values. Nested overrides are not allowed.
 class ScopedMockClockOverride {
  public:
   ScopedMockClockOverride();