Fix Linux64_TSAN-concurrent_marking bot after 7a9e3ec9

The mock histogram functions cannot be cleared and can be called on
isolate tear down if incremental marking is in progress.

Bug: chromium:850508
Tbr: mlippautz@chromium.org
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Change-Id: I99e52aaa81c863f71e195aeed691b37da9e71da6
Reviewed-on: https://chromium-review.googlesource.com/1093073Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53616}

test/unittests/heap/gc-tracer-unittest.cc

@@ -460,6 +460,7 @@ class GcHistogram {
   }
 
   static void AddHistogramSample(void* histogram, int sample) {
+    if (histograms_.empty()) return;
     static_cast<GcHistogram*>(histogram)->samples_.push_back(sample);
   }
 
@@ -486,6 +487,7 @@ std::map<std::string, std::unique_ptr<GcHistogram>> GcHistogram::histograms_ =
     std::map<std::string, std::unique_ptr<GcHistogram>>();
 
 TEST_F(GCTracerTest, RecordMarkCompactHistograms) {
+  if (FLAG_stress_incremental_marking) return;
   isolate()->SetCreateHistogramFunction(&GcHistogram::CreateHistogram);
   isolate()->SetAddHistogramSampleFunction(&GcHistogram::AddHistogramSample);
   GCTracer* tracer = i_isolate()->heap()->tracer();