Add support for mojo::TypeConverter to control how convenient conversion should be.

mojo/services/native_viewport/geometry_conversions.h

 // 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.
 
 #ifndef MOJO_SERVICES_NATIVE_VIEWPORT_GEOMETRY_CONVERSIONS_H_
 #define MOJO_SERVICES_NATIVE_VIEWPORT_GEOMETRY_CONVERSIONS_H_
 
 #include "mojo/services/native_viewport/native_viewport.mojom.h"
 #include "ui/gfx/rect.h"
 
 namespace mojo {
 
 template<>
 class TypeConverter<Point, gfx::Point> {
 public:
   static Point ConvertFrom(const gfx::Point& input, Buffer* buf) {
     Point::Builder point(buf);
     point.set_x(input.x());
     point.set_y(input.y());
     return point.Finish();
   }
   static gfx::Point ConvertTo(const Point& input) {
     return gfx::Point(input.x(), input.y());
   }
+
+  MOJO_ALLOW_IMPLICIT_TYPE_CONVERSION();
 };
 
 template<>
 class TypeConverter<Size, gfx::Size> {
 public:
   static Size ConvertFrom(const gfx::Size& input, Buffer* buf) {
     Size::Builder size(buf);
     size.set_width(input.width());
     size.set_height(input.height());
     return size.Finish();
   }
   static gfx::Size ConvertTo(const Size& input) {
     return gfx::Size(input.width(), input.height());
   }
+
+  MOJO_ALLOW_IMPLICIT_TYPE_CONVERSION();
 };
 
 template<>
 class TypeConverter<Rect, gfx::Rect> {
  public:
   static Rect ConvertFrom(const gfx::Rect& input, Buffer* buf) {
     Rect::Builder rect(buf);
     rect.set_position(input.origin());
     rect.set_size(input.size());
     return rect.Finish();
   }
   static gfx::Rect ConvertTo(const Rect& input) {
     return gfx::Rect(input.position().x(), input.position().y(),
                      input.size().width(), input.size().height());
   }
+
+  MOJO_ALLOW_IMPLICIT_TYPE_CONVERSION();
 };
 
 }  // namespace mojo
 
 #endif  // MOJO_SERVICES_NATIVE_VIEWPORT_GEOMETRY_CONVERSIONS_H_