Copy DefaultTickClock's usage note to TickClock

Per the note, you probably don't need to use TickClock or
DefaultTickClock anymore. Stick that note in both TickClock and
DefaultTickClockheaders instead of just DefaultTickClock.

Bug: None
Test: None
Change-Id: I7cdaa5b198ad06b230e2b0406c51f49f769b0794
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2547749
Commit-Queue: David Munro <davidmunro@google.com>
Auto-Submit: David Munro <davidmunro@google.com>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#830072}

base/time/default_tick_clock.h

@@ -13,9 +13,9 @@ namespace base {
 // DefaultTickClock is a TickClock implementation that uses TimeTicks::Now().
 // This is typically used by components that expose a SetTickClockForTesting().
 // Note: Overriding Time/TimeTicks altogether via
-// TaskEnvironment::TimeSource::MOCK_TIME is now the preferred of
-// overriding time in unit tests. As such, there shouldn't be many new use cases
-// for TickClock/DefaultTickClock anymore.
+// TaskEnvironment::TimeSource::MOCK_TIME is now the preferred way of overriding
+// time in unit tests. As such, there shouldn't be many new use cases for
+// TickClock/DefaultTickClock anymore.
 class BASE_EXPORT DefaultTickClock : public TickClock {
  public:
   ~DefaultTickClock() override;

base/time/tick_clock.h

@@ -14,6 +14,11 @@ namespace base {
 // intended to be able to test the behavior of classes with respect to
 // non-decreasing time.
 //
+// Note: Overriding Time/TimeTicks altogether via
+// TaskEnvironment::TimeSource::MOCK_TIME is now the preferred way of overriding
+// time in unit tests. As such, there shouldn't be many new use cases for
+// TickClock/DefaultTickClock anymore.
+//
 // See DefaultTickClock (base/time/default_tick_clock.h) for the default
 // implementation that simply uses TimeTicks::Now().
 //