Make the MediaRouterDialogDelegate set initial dialog size in OSX

chrome/browser/ui/cocoa/constrained_window/constrained_window_web_dialog_sheet.mm

 // 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.
 
 #import "chrome/browser/ui/cocoa/constrained_window/constrained_window_web_dialog_sheet.h"
 
 #include "ui/gfx/geometry/size.h"
 #include "ui/web_dialogs/web_dialog_delegate.h"
 
 @implementation WebDialogConstrainedWindowSheet
 
 - (id)initWithCustomWindow:(NSWindow*)customWindow
          webDialogDelegate:(ui::WebDialogDelegate*)delegate {
   if (self = [super initWithCustomWindow:customWindow]) {
     web_dialog_delegate_ = delegate;
   }
 
   return self;
 }
 
 - (void)updateSheetPosition {
   if (web_dialog_delegate_) {
-    gfx::Size size;
+    // If the dialog has autoresizing enabled and |current_size_| has been set,
+    // GetDialogSize() doesn't modify |size|.
+    gfx::Size size(current_size_.width, current_size_.height);
     web_dialog_delegate_->GetDialogSize(&size);
+    DCHECK(!size.IsEmpty());
 
-    // If the dialog has autoresizing enabled, |size| will be empty. Use the
-    // last known dialog size.
-    NSSize content_size = size.IsEmpty() ? current_size_ :
-        NSMakeSize(size.width(), size.height());
+    NSSize content_size = NSMakeSize(size.width(), size.height());
     [customWindow_ setContentSize:content_size];
   }
   [super updateSheetPosition];
 }
 
 - (void)resizeWithNewSize:(NSSize)size {
   current_size_ = size;
   [customWindow_ setContentSize:current_size_];
 
   // self's updateSheetPosition() sets |customWindow_|'s contentSize to a
   // fixed dialog size. Here, we want to resize to |size| instead. Use
   // super rather than self to bypass the setContentSize() call for the fixed
   // size.
   [super updateSheetPosition];
 }
 
 @end