Add a pure virtual method to the GarbageCollectedMixin class

Source/core/dom/DocumentParserClient.h

Index: Source/core/dom/DocumentParserClient.h
diff --git a/Source/core/dom/DocumentParserClient.h b/Source/core/dom/DocumentParserClient.h
index 38cc39bc9017c8ed33bb9f4e15c5439f559c6352..05a94a2a74fde2033e14d1a1fcc3d2050880a1f7 100644
--- a/Source/core/dom/DocumentParserClient.h
+++ b/Source/core/dom/DocumentParserClient.h
@@ -14,6 +14,8 @@ public:
     // This callback is called when all data pushed to parser has been consumed.
     virtual void notifyParserStopped() = 0;
 
+    virtual void trace(Visitor*) { }
+
 protected:
     DocumentParserClient() { }
 };