A background task can now use GCTracer::BackgroundScope to
trace the time spent in the task. The time shows up in
--trace-gc-nvp output and in the runtime call stats for GC.

The destructor of GCTracer::BackgroundScope increments the
corresponding counter in heap()->tracer()->background_counter_,
which is protected by a mutex.

The GCTracer::Stop function fetches background_counter_ items
into the global scope and into the runtime call stats.

Bug: chromium:758183
Change-Id: Id7bcd5089ba6c027fe9a57eb3f7db1cb5092aec5
Reviewed-on: https://chromium-review.googlesource.com/801694
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49841}