Disable SamplingHeapProfilerTest.StartStop on Mac.

The test is crashing in a flaky manner.

Bug: 1116543
Change-Id: I9c0d020aa8a936be1ffce98f207a892f2a0516bf
Tbr: erikchen@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2357253Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Erik Chen <erikchen@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#798232}

base/sampling_heap_profiler/sampling_heap_profiler_unittest.cc

@@ -270,7 +270,13 @@ class StartStopThread : public SimpleThread {
   WaitableEvent* event_;
 };
 
-TEST_F(SamplingHeapProfilerTest, StartStop) {
+// Flaky on Mac. crbug.com/1116543
+#if defined(OS_MAC)
+#define MAYBE_StartStop DISABLED_StartStop
+#else
+#define MAYBE_StartStop StartStop
+#endif
+TEST_F(SamplingHeapProfilerTest, MAYBE_StartStop) {
   auto* profiler = SamplingHeapProfiler::Get();
   EXPECT_EQ(0, GetRunningSessionsCount());
   profiler->Start();