This adds GN configs for using sanitizer coverage and refactors gcov
configs. Now, both coverage kinds are behind the same gn argument
v8_code_coverage.

This also switches the bot to GN.

Bug: chromium:645890,v8:5502
Change-Id: I3af606e0cad109dd790a121d0d92e53cf4e38f9a
Reviewed-on: https://chromium-review.googlesource.com/549360Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46254}