Convert ReadData...() to use the new user pointer handling (see r285350).

mojo/system/memory.h

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef MOJO_SYSTEM_MEMORY_H_
 #define MOJO_SYSTEM_MEMORY_H_
 
 #include <stddef.h>
 #include <stdint.h>
 #include <string.h>  // For |memcpy()|.
@@ skipping 163 matching lines @@
   // |memcpy(user_pointer, source, count * sizeof(Type))|.
   //
   // Note: The same comments about the validity of |Put()| (except for the part
   // about |void|) apply here.
   void PutArray(const Type* source, size_t count) {
     internal::CheckUserPointerWithCountHelper<
         sizeof(NonVoidType), MOJO_ALIGNOF(NonVoidType)>(pointer_, count);
     memcpy(pointer_, source, count * sizeof(NonVoidType));
   }
 
-  // Gets a |UserPointer| at offset |i| (in |Type|s) relative to this. This
-  // method is not valid if |Type| is |void| (TODO(vtl): Maybe I should make it
-  // valid, with the offset in bytes?).
+  // Gets a |UserPointer| at offset |i| (in |Type|s) relative to this.
   UserPointer At(size_t i) const {
-    return UserPointer(pointer_ + i);
+    return UserPointer(static_cast<Type*>(
+        static_cast<NonVoidType*>(pointer_) + i));
   }
 
   // TODO(vtl): This is temporary. Get rid of this. (We should pass
   // |UserPointer<>|s along appropriately, or access data in a safe way
   // everywhere.)
   Type* GetPointerUnsafe() const {
     return pointer_;
   }
 
   // These provides safe (read-only/write-only/read-and-write) access to a
@@ skipping 158 matching lines @@
 // Provides a convenient way to make a |UserPointerValue<Type>|.
 template <typename Type>
 inline UserPointerValue<Type> MakeUserPointerValue(Type* pointer) {
   return UserPointerValue<Type>(pointer);
 }
 
 }  // namespace system
 }  // namespace mojo
 
 #endif  // MOJO_SYSTEM_MEMORY_H_