Add a few missing overrides found by a new clang warning.

src/ic/ic.h

Index: src/ic/ic.h
===================================================================
--- src/ic/ic.h	(revision 24959)
+++ src/ic/ic.h	(working copy)
@@ -416,7 +416,7 @@
 
   virtual Handle<Code> CompileHandler(LookupIterator* lookup,
                                       Handle<Object> unused,
-                                      CacheHolderFlag cache_holder);
+                                      CacheHolderFlag cache_holder) OVERRIDE;
 
  private:
   virtual Handle<Code> pre_monomorphic_stub() const;
@@ -545,7 +545,7 @@
                     JSReceiver::StoreFromKeyed store_mode);
   virtual Handle<Code> CompileHandler(LookupIterator* lookup,
                                       Handle<Object> value,
-                                      CacheHolderFlag cache_holder);
+                                      CacheHolderFlag cache_holder) OVERRIDE;
 
  private:
   inline void set_target(Code* code);