[threadpool] Disable flaky ThreadGroupImplHistogramTest test.

TBR=gab

Bug: 1047733
Change-Id: Iffdf9abbb726e05fddf3e746b39f8ec461c4f616
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2051988Reviewed-by: Wez <wez@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#740651}

base/task/thread_pool/thread_group_impl_unittest.cc

@@ -617,7 +617,8 @@ class ThreadGroupImplHistogramTest : public ThreadGroupImplImplTest {
 
 }  // namespace
 
-TEST_F(ThreadGroupImplHistogramTest, NumTasksBeforeCleanup) {
+// TODO(https://crbug.com/1047733): Flaky, especially on Windows and Android.
+TEST_F(ThreadGroupImplHistogramTest, DISABLED_NumTasksBeforeCleanup) {
   CreateThreadGroup();
   auto histogrammed_thread_task_runner = test::CreatePooledSequencedTaskRunner(
       {WithBaseSyncPrimitives()}, &mock_pooled_task_runner_delegate_);