[cctest] Add one more GC to a flaky test

It now passes on both 32-bit and 64-bit nosnap bots.

TBR=ulan@chromium.org
NOTREECHECKS=true

Change-Id: Id797c88f1eb32868433e112883c2c64b8640eb2c
Reviewed-on: https://chromium-review.googlesource.com/489682Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44953}

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

@@ -313,6 +313,8 @@ UNINITIALIZED_TEST(ArrayBuffer_SemiSpaceCopyMultipleTasks) {
     // Ensure heap is in a clean state.
     heap->CollectAllGarbage(Heap::kFinalizeIncrementalMarkingMask,
                             GarbageCollectionReason::kTesting);
+    heap->CollectAllGarbage(Heap::kFinalizeIncrementalMarkingMask,
+                            GarbageCollectionReason::kTesting);
 
     Local<v8::ArrayBuffer> ab1 = v8::ArrayBuffer::New(isolate, 100);
     Handle<JSArrayBuffer> buf1 = v8::Utils::OpenHandle(*ab1);