Fix HashSetTest.HashSetRefPtr test not to be flaky.

The test assumes specific values of DummyRefCounted::ref_invokes_count_
without re-setting it to zero at the start of the test.

TBR: haraken
Change-Id: I265fad41cbfa5b8692a7f620d4fa253736d32c74
Reviewed-on: https://chromium-review.googlesource.com/1188701Reviewed-by: Wez <wez@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#586151}

third_party/blink/renderer/platform/wtf/hash_set_test.cc

@@ -197,6 +197,8 @@ TEST(HashSetTest, HashSetOwnPtr) {
 
 TEST(HashSetTest, HashSetRefPtr) {
   bool is_deleted = false;
+  DummyRefCounted::ref_invokes_count_ = 0;
+
   scoped_refptr<DummyRefCounted> ptr =
       base::AdoptRef(new DummyRefCounted(is_deleted));
   EXPECT_EQ(0, DummyRefCounted::ref_invokes_count_);