heap: Add concurrent marking flag to UnifiedHeapMarkingVisitor.

This flag will be used by https://crrev.com/c/1730054.
Submitting as separate CL so that if 1730054 needs to be reverted, we
still have the flag in the code to use in further CLs.

Bug: 986235
Change-Id: I3300937e864f6e3fcd40d6c0a463fae04911e71e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1725643Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#684323}

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

@@ -4,6 +4,7 @@
 
 #include "third_party/blink/renderer/platform/heap/unified_heap_marking_visitor.h"
 
+#include "third_party/blink/public/common/features.h"
 #include "third_party/blink/renderer/platform/bindings/script_wrappable.h"
 #include "third_party/blink/renderer/platform/bindings/trace_wrapper_v8_reference.h"
 #include "third_party/blink/renderer/platform/bindings/v8_per_isolate_data.h"
@@ -18,7 +19,9 @@ UnifiedHeapMarkingVisitorBase::UnifiedHeapMarkingVisitorBase(
     v8::Isolate* isolate)
     : MarkingVisitor(thread_state, mode),
       isolate_(isolate),
-      controller_(thread_state->unified_heap_controller()) {
+      controller_(thread_state->unified_heap_controller()),
+      is_concurrent_marking_enabled_(base::FeatureList::IsEnabled(
+          blink::features::kBlinkHeapConcurrentMarking)) {
   DCHECK(controller_);
 }
 

third_party/blink/renderer/platform/heap/unified_heap_marking_visitor.h

@@ -27,6 +27,10 @@ class PLATFORM_EXPORT UnifiedHeapMarkingVisitorBase : public MarkingVisitor {
   // Visitation methods that announce reachable wrappers to V8.
   void Visit(const TraceWrapperV8Reference<v8::Value>&) final;
 
+  bool is_concurrent_marking_enabled() const {
+    return is_concurrent_marking_enabled_;
+  }
+
  protected:
   UnifiedHeapMarkingVisitorBase(ThreadState*, MarkingMode, v8::Isolate*);
 
@@ -34,6 +38,9 @@ class PLATFORM_EXPORT UnifiedHeapMarkingVisitorBase : public MarkingVisitor {
   v8::EmbedderHeapTracer* const controller_;
 
   DISALLOW_COPY_AND_ASSIGN(UnifiedHeapMarkingVisitorBase);
+
+ private:
+  bool is_concurrent_marking_enabled_;
 };
 
 // Same as the base visitor with the difference that it is bound to main thread.