blink/heap: ensure finalizer tests are repeatable

The static |destructor_calls_| variables used with the
HeapTest.Finalization and HeapTest.SimpleFinalization tests were not
being reset to 0 on subsequent runs of the test
(e.g. when running with --gtest_repeat).

BUG=899757

Change-Id: I55419a946431267cf88122aae80ae46fcbe8f467
Reviewed-on: https://chromium-review.googlesource.com/c/1309693Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604201}

third_party/blink/renderer/platform/heap/heap_test.cc

@@ -1901,6 +1901,7 @@ TEST(HeapTest, SimplePersistent) {
 
 TEST(HeapTest, SimpleFinalization) {
   {
+    SimpleFinalizedObject::destructor_calls_ = 0;
     Persistent<SimpleFinalizedObject> finalized =
         SimpleFinalizedObject::Create();
     EXPECT_EQ(0, SimpleFinalizedObject::destructor_calls_);
@@ -2072,6 +2073,8 @@ TEST(HeapTest, EagerlySweepingPages) {
 
 TEST(HeapTest, Finalization) {
   {
+    HeapTestSubClass::destructor_calls_ = 0;
+    HeapTestSuperClass::destructor_calls_ = 0;
     HeapTestSubClass* t1 = HeapTestSubClass::Create();
     HeapTestSubClass* t2 = HeapTestSubClass::Create();
     HeapTestSuperClass* t3 = HeapTestSuperClass::Create();