Add support for IncrementCounter in Hydrogen.

src/assembler.h

Index: src/assembler.h
diff --git a/src/assembler.h b/src/assembler.h
index 481add56b8df37685b2ef16eee3a0eb6603c8fea..36aaf4d70b9c2a8e920db208d0d66952abfc2974 100644
--- a/src/assembler.h
+++ b/src/assembler.h
@@ -829,7 +829,7 @@ class ExternalReference BASE_EMBEDDED {
 
   static ExternalReference cpu_features();
 
-  Address address() const {return reinterpret_cast<Address>(address_);}
+  Address address() const { return reinterpret_cast<Address>(address_); }
 
 #ifdef ENABLE_DEBUGGER_SUPPORT
   // Function Debug::Break()
@@ -868,6 +868,16 @@ class ExternalReference BASE_EMBEDDED {
 
   static ExternalReference stress_deopt_count(Isolate* isolate);
 
+  static ExternalReference null();
+
+  bool operator==(const ExternalReference& other) const {
+    return address_ == other.address_;
+  }
+
+  bool operator!=(const ExternalReference& other) const {
+    return !(*this == other);
+  }
+
  private:
   explicit ExternalReference(void* address)
       : address_(address) {}