[MacViews] Position modal sheets using ModalDialogHost.

ui/views/cocoa/views_nswindow_delegate.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 "ui/views/cocoa/views_nswindow_delegate.h"
 
 #include "base/logging.h"
 #import "ui/views/cocoa/bridged_content_view.h"
 #import "ui/views/cocoa/bridged_native_widget.h"
 #include "ui/views/widget/native_widget_mac.h"
@@ skipping 112 matching lines @@
 // Allow non-resizable windows (without NSResizableWindowMask) to fill the
 // screen in fullscreen mode. This only happens when
 // -[NSWindow toggleFullscreen:] is called since non-resizable windows have no
 // fullscreen button. Without this they would only enter fullscreen at their
 // current size.
 - (NSSize)window:(NSWindow*)window
     willUseFullScreenContentSize:(NSSize)proposedSize {
   return proposedSize;
 }
 
+// Override to correctly position modal dialogs.
+- (NSRect)window:(NSWindow*)window
+    willPositionSheet:(NSWindow*)sheet
+            usingRect:(NSRect)defaultSheetLocation {
+  // As per NSWindowDelegate documentation, the origin indicates the top left
+  // point of the host frame in window coordinates. The width changes the
+  // animation from vertical to trapezoid if it is smaller than the width of the
+  // dialog. The height is ignored but should be set to zero.
+  return NSMakeRect(0, [self nativeWidgetMac]->SheetPositionY(),
+                    NSWidth(defaultSheetLocation), 0);
+}
+
 @end