Add const to two methods.

gfx/point.h

 // Copyright (c) 2010 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 GFX_POINT_H_
 #define GFX_POINT_H_
 
 #include "build/build_config.h"
 
 #include <iosfwd>
@@ skipping 34 matching lines @@
   }
 
   void set_x(int x) { x_ = x; }
   void set_y(int y) { y_ = y; }
 
   void Offset(int delta_x, int delta_y) {
     x_ += delta_x;
     y_ += delta_y;
   }
 
-  Point Add(const Point& other) {
+  Point Add(const Point& other) const{
     Point copy = *this;
     copy.Offset(other.x_, other.y_);
     return copy;
   }
 
-  Point Subtract(const Point& other) {
+  Point Subtract(const Point& other) const {
     Point copy = *this;
     copy.Offset(-other.x_, -other.y_);
     return copy;
   }
 
   bool operator==(const Point& rhs) const {
     return x_ == rhs.x_ && y_ == rhs.y_;
   }
 
   bool operator!=(const Point& rhs) const {
     return !(*this == rhs);
   }
 
 #if defined(OS_WIN)
   POINT ToPOINT() const;
 #elif defined(OS_MACOSX)
   CGPoint ToCGPoint() const;
 #endif
 
  private:
   int x_;
   int y_;
 };
 
 }  // namespace gfx
 
 std::ostream& operator<<(std::ostream& out, const gfx::Point& p);
 
 #endif  // GFX_POINT_H_