[heap] Make GCTracer::ResetForTesting thread safe.

Bug: 
Change-Id: I49a259b8911969aace193cc3d0b18e4b8bcac7b8
Reviewed-on: https://chromium-review.googlesource.com/818344Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49988}

src/heap/gc-tracer.cc

@@ -172,6 +172,7 @@ void GCTracer::ResetForTesting() {
   recorded_context_disposal_times_.Reset();
   recorded_survival_ratios_.Reset();
   start_counter_ = 0;
+  base::LockGuard<base::Mutex> guard(&background_counter_mutex_);
   for (int i = 0; i < BackgroundScope::NUMBER_OF_SCOPES; i++) {
     background_counter_[i].total_duration_ms = 0;
     background_counter_[i].runtime_call_counter.Reset();