Mark Local::As and Persistent::As as const

Since they are.

R=verwaest@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2216123003
Cr-Commit-Position: refs/heads/master@{#38386}

include/v8.h

@@ -292,8 +292,8 @@ class Local {
     return Local<T>(T::Cast(*that));
   }
 
-
-  template <class S> V8_INLINE Local<S> As() {
+  template <class S>
+  V8_INLINE Local<S> As() const {
     return Local<S>::Cast(*this);
   }
 
@@ -743,17 +743,18 @@ template <class T, class M> class Persistent : public PersistentBase<T> {
 
   // TODO(dcarney): this is pretty useless, fix or remove
   template <class S>
-  V8_INLINE static Persistent<T>& Cast(Persistent<S>& that) { // NOLINT
+  V8_INLINE static Persistent<T>& Cast(const Persistent<S>& that) {  // NOLINT
 #ifdef V8_ENABLE_CHECKS
     // If we're going to perform the type check then we have to check
     // that the handle isn't empty before doing the checked cast.
     if (!that.IsEmpty()) T::Cast(*that);
 #endif
-    return reinterpret_cast<Persistent<T>&>(that);
+    return reinterpret_cast<Persistent<T>&>(const_cast<Persistent<S>&>(that));
   }
 
   // TODO(dcarney): this is pretty useless, fix or remove
-  template <class S> V8_INLINE Persistent<S>& As() { // NOLINT
+  template <class S>
+  V8_INLINE Persistent<S>& As() const {  // NOLINT
     return Persistent<S>::Cast(*this);
   }