[cctest] Fix more flakiness in Heap-related cctests on nosnap builds

These tests assume that a newly-created Isolate is pristine, but that's
not true for nosnap builds.

TBR=ulan@chromium.org

Change-Id: Ie5d0fb0450f285c8eeb8e088feef6729102c0f14
Reviewed-on: https://chromium-review.googlesource.com/489063Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44952}

test/cctest/heap/test-array-buffer-tracker.cc

@@ -310,6 +310,10 @@ UNINITIALIZED_TEST(ArrayBuffer_SemiSpaceCopyMultipleTasks) {
     v8::Context::New(isolate)->Enter();
     Heap* heap = i_isolate->heap();
 
+    // Ensure heap is in a clean state.
+    heap->CollectAllGarbage(Heap::kFinalizeIncrementalMarkingMask,
+                            GarbageCollectionReason::kTesting);
+
     Local<v8::ArrayBuffer> ab1 = v8::ArrayBuffer::New(isolate, 100);
     Handle<JSArrayBuffer> buf1 = v8::Utils::OpenHandle(*ab1);
     heap::FillCurrentPage(heap->new_space());

test/cctest/heap/test-heap.cc

@@ -2106,8 +2106,7 @@ TEST(GrowAndShrinkNewSpace) {
   }
 
   // Make sure we're in a consistent state to start out.
-  CcTest::CollectGarbage(NEW_SPACE);
-  CcTest::CollectGarbage(NEW_SPACE);
+  CcTest::CollectAllGarbage();
 
   // Explicitly growing should double the space capacity.
   size_t old_capacity, new_capacity;