[heap] Fix StartBlackAllocationForTesting.

The function shouldn't try to start black allocation if it is already
started.

Bug: chromium:694255
TBR: mlippautz@chromium.org
Change-Id: I77b5346f6ac2ec5947ca4351a8abe33865729fda
Reviewed-on: https://chromium-review.googlesource.com/612385Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47334}

src/heap/incremental-marking.cc

@@ -584,6 +584,7 @@ void IncrementalMarking::StartMarking() {
 
 void IncrementalMarking::StartBlackAllocation() {
   DCHECK(FLAG_black_allocation);
+  DCHECK(!black_allocation_);
   DCHECK(IsMarking());
   black_allocation_ = true;
   heap()->old_space()->MarkAllocationInfoBlack();

src/heap/incremental-marking.h

@@ -253,7 +253,11 @@ class V8_EXPORT_PRIVATE IncrementalMarking {
 
   bool black_allocation() { return black_allocation_; }
 
-  void StartBlackAllocationForTesting() { StartBlackAllocation(); }
+  void StartBlackAllocationForTesting() {
+    if (!black_allocation_) {
+      StartBlackAllocation();
+    }
+  }
 
   void AbortBlackAllocation();