Mojo C++ bindings: share DataView class between chromium and blink variants.

mojo/public/interfaces/bindings/tests/rect.mojom

 // Copyright 2014 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.
 
 [JavaPackage="org.chromium.mojo.bindings.test.mojom.test_structs"]
 module mojo.test;
 
 struct Rect {
   int32 x;
   int32 y;
   int32 width;
   int32 height;
 };
 
-// A copy of Rect that can be typemapped. Arrays of Rect are currently used,
-// which do not support typemapping.
+// A copy of Rect that is typemapped differently in the chromium and blink
+// variants.
 struct TypemappedRect {
   int32 x;
   int32 y;
   int32 width;
   int32 height;
 };
+
+// A copy of Rect that is typemapped to the same custom type in the chromium and
+// blink variants.
+struct SharedTypemappedRect {
+  int32 x;
+  int32 y;
+  int32 width;
+  int32 height;
+};