Mojo C++ bindings: make String16 and gfx::Size available in Blink

mojo/common/common_custom_types.mojom

 // Copyright 2016 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.
 
 module mojo.common.mojom;
 
 [Native]
 struct FilePath;
 
 [Native]
 struct ListValue;
 
 [Native]
 struct DictionaryValue;
 
 [Native]
 struct Time;
 
 struct TimeDelta {
   int64 microseconds;
 };
 
 [Native]
 struct TimeTicks;
 
-[Native]
-struct String16;
+// Corresponds to |base::string16| in base/strings/string16.h
+// Corresponds to |WTF::String| in
+// third_party/WebKit/Source/wtf/text/WTFString.h.
+struct String16 {
+  array<uint16> data;
+};
 
 // Corresponds to |base::UnguessableToken| in base/unguessable_token.h
 struct UnguessableToken {
   uint64 high;
   uint64 low;
 };
 
 // Corresponds to |base::Version| in base/version.h
 struct Version {
   array<uint32> components;
 };