Move preferred scrollbar style preference change listening from renderer to browser, 4 of 3.

content/browser/theme_helper_mac.mm

 // Copyright 2013 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 "content/browser/theme_helper_mac.h"
 
 #import <Cocoa/Cocoa.h>
 
+#include "base/command_line.h"
 #include "content/common/view_messages.h"
 #include "content/public/browser/browser_thread.h"
 #include "content/public/browser/notification_service.h"
 #include "content/public/browser/notification_types.h"
 #include "content/public/browser/render_process_host.h"
+#include "content/public/common/content_switches.h"
 
 // Declare notification names from the 10.7 SDK.
 #if !defined(MAC_OS_X_VERSION_10_7) || \
     MAC_OS_X_VERSION_MAX_ALLOWED < MAC_OS_X_VERSION_10_7
 NSString* NSPreferredScrollerStyleDidChangeNotification =
     @"NSPreferredScrollerStyleDidChangeNotification";
 
 @interface NSScroller (LionSDK)
 + (NSInteger)preferredScrollerStyle;
 @end
@@ skipping 18 matching lines @@
            object:nil
 suspensionBehavior:NSNotificationSuspensionBehaviorDeliverImmediately];
 
   [[NSDistributedNotificationCenter defaultCenter]
       addObserver:self
          selector:@selector(behaviorPrefsChanged:)
              name:@"AppleNoRedisplayAppearancePreferenceChanged"
            object:nil
 suspensionBehavior:NSNotificationSuspensionBehaviorCoalesce];
 
-  if ([NSScroller respondsToSelector:@selector(preferredScrollerStyle)]) {
+  // In single-process mode, renderers will catch these notifications
+  // themselves and listening for them here may trigger the DCHECK in Observe().
+  if ([NSScroller respondsToSelector:@selector(preferredScrollerStyle)] &&
+      !CommandLine::ForCurrentProcess()->HasSwitch(switches::kSingleProcess)) {
     [[NSNotificationCenter defaultCenter]
         addObserver:self
            selector:@selector(behaviorPrefsChanged:)
                name:NSPreferredScrollerStyleDidChangeNotification
              object:nil];
   }
 }
 
 + (void)appearancePrefsChanged:(NSNotification*)notification {
   [self notifyPrefsChangedWithRedraw:YES];
@@ skipping 74 matching lines @@
   RenderProcessHost* rph = Source<RenderProcessHost>(source).ptr();
   rph->Send(new ViewMsg_UpdateScrollbarTheme(
       [defaults floatForKey:@"NSScrollerButtonDelay"],
       [defaults floatForKey:@"NSScrollerButtonPeriod"],
       [defaults boolForKey:@"AppleScrollerPagingBehavior"],
       GetPreferredScrollerStyle(),
       false));
 }
 
 }  // namespace content