Avoid virtual call on the fast path when invoking
Heap::MonotonicallyIncreasingTimeInMs by using base::TimeTicks directly.
With --predictable we need to fall back to
Heap::MonotonicallyIncreasingTimeInMs though.

Convert base::TimeTicks to milliseconds as double, since this was
the format GCTracer was using internally.

Change-Id: I25ab6f1a77ffc8a317782b5754521c39ccb46e48
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2562240Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71440}