[heap] Add GN flag for enabling concurrent marking.

src/heap/concurrent-marking.cc

 // Copyright 2017 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/heap/concurrent-marking.h"
 
 #include <stack>
 #include <unordered_map>
 
 #include "src/heap/concurrent-marking-deque.h"
@@ skipping 218 matching lines @@
   base::Semaphore* on_finish_;
   DISALLOW_COPY_AND_ASSIGN(Task);
 };
 
 ConcurrentMarking::ConcurrentMarking(Heap* heap, ConcurrentMarkingDeque* deque)
     : heap_(heap),
       pending_task_semaphore_(0),
       deque_(deque),
       visitor_(new ConcurrentMarkingVisitor(deque_)),
       is_task_pending_(false) {
-  // Concurrent marking does not work with double unboxing.
-  STATIC_ASSERT(!(V8_CONCURRENT_MARKING && V8_DOUBLE_FIELDS_UNBOXING));
   // The runtime flag should be set only if the compile time flag was set.
-  CHECK(!FLAG_concurrent_marking || V8_CONCURRENT_MARKING);
+#ifndef V8_CONCURRENT_MARKING
+  CHECK(!FLAG_concurrent_marking);
+#endif
 }
 
 ConcurrentMarking::~ConcurrentMarking() { delete visitor_; }
 
 void ConcurrentMarking::Run() {
   double time_ms = heap_->MonotonicallyIncreasingTimeInMs();
   size_t bytes_marked = 0;
   base::Mutex* relocation_mutex = heap_->relocation_mutex();
   {
     TimedScope scope(&time_ms);
@@ skipping 25 matching lines @@
 }
 
 void ConcurrentMarking::EnsureTaskCompleted() {
   if (IsTaskPending()) {
     WaitForTaskToComplete();
   }
 }
 
 }  // namespace internal
 }  // namespace v8