Move SafeStackFrameIterator::active_count_ into an isolate.

src/frames.h

Index: src/frames.h
diff --git a/src/frames.h b/src/frames.h
index bee95ccbf1d2582c448a344280429cc6e07f7bdf..e95229800e5919f365edd50566f590512c8cc0bc 100644
--- a/src/frames.h
+++ b/src/frames.h
@@ -739,7 +739,7 @@ class SafeStackFrameIterator BASE_EMBEDDED {
   void Advance();
   void Reset();
 
-  static bool is_active() { return active_count_ > 0; }
+  static bool is_active(Isolate* isolate);
 
   static bool IsWithinBounds(
       Address low_bound, Address high_bound, Address addr) {
@@ -786,13 +786,13 @@ class SafeStackFrameIterator BASE_EMBEDDED {
   // heap objects.
   class ActiveCountMaintainer BASE_EMBEDDED {
    public:
-    ActiveCountMaintainer() { active_count_++; }
-    ~ActiveCountMaintainer() { active_count_--; }
+    explicit ActiveCountMaintainer(Isolate* isolate);
+    ~ActiveCountMaintainer();
+   private:
+    Isolate* isolate_;
   };
 
   ActiveCountMaintainer maintainer_;
-  // TODO(isolates): this is dangerous.
-  static int active_count_;
   StackAddressValidator stack_validator_;
   const bool is_valid_top_;
   const bool is_valid_fp_;