Adds a base class TestPlatform which implements the most common defaults
for v8::Platform methods.

Reworks existing cctests and unittests to use TestPlatform.

Bug: 
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: Ifeb28a5a190529697d5bcac227e80b10d454d9bd
Reviewed-on: https://chromium-review.googlesource.com/590194Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47050}