views: Also const-ify GetMaximumSize().

ui/views/window/native_frame_view.cc

 // Copyright (c) 2011 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.
 
 #include "ui/views/window/native_frame_view.h"
 
 #include "ui/views/widget/native_widget.h"
 #include "ui/views/widget/widget.h"
 
 #if defined(OS_WIN)
@@ skipping 70 matching lines @@
 #else
   return frame_->non_client_view()->GetWindowBoundsForClientBounds(
       gfx::Rect(client_preferred_size)).size();
 #endif
 }
 
 gfx::Size NativeFrameView::GetMinimumSize() const {
   return frame_->client_view()->GetMinimumSize();
 }
 
-gfx::Size NativeFrameView::GetMaximumSize() {
+gfx::Size NativeFrameView::GetMaximumSize() const {
   return frame_->client_view()->GetMaximumSize();
 }
 
 const char* NativeFrameView::GetClassName() const {
   return kViewClassName;
 }
 
 }  // namespace views