Make it possible for embedders to provide their own tracing timetamps by
providing an overridable virtual function on V8's tracing controller.

Bug: 
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: I727e633cb7f63d4b41c2e427ecca3c9174c90bfe
Reviewed-on: https://chromium-review.googlesource.com/847690Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Fadi Meawad <fmeawad@chromium.org>
Commit-Queue: Ali Ijaz Sheikh <ofrobots@google.com>
Cr-Commit-Position: refs/heads/master@{#50489}