Fix a few warnings that -Wnull-conversion of a future clang will complain about.

content/browser/renderer_host/render_widget_host_view_mac.mm

 // Copyright (c) 2012 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/renderer_host/render_widget_host_view_mac.h"
 
 #include <QuartzCore/QuartzCore.h>
 
 #include "base/bind.h"
 #include "base/debug/trace_event.h"
@@ skipping 2076 matching lines @@
   // accessibilityPerformShowMenuAction.
   NSPoint location = [self accessibilityPointInScreen:accessibility];
   NSSize size = [[accessibility size] sizeValue];
   location = [[self window] convertScreenToBase:location];
   location.x += size.width/2;
   location.y += size.height/2;
 
   NSEvent* fakeRightClick = [NSEvent
                            mouseEventWithType:NSRightMouseDown
                                      location:location
-                                modifierFlags:nil
+                                modifierFlags:0
                                     timestamp:0
                                  windowNumber:[[self window] windowNumber]
                                       context:[NSGraphicsContext currentContext]
                                   eventNumber:0
                                    clickCount:1
                                      pressure:0];
 
   [self mouseEvent:fakeRightClick];
 }
 
@@ skipping 676 matching lines @@
   if (!string) return NO;
 
   // If the user is currently using an IME, confirm the IME input,
   // and then insert the text from the service, the same as TextEdit and Safari.
   [self confirmComposition];
   [self insertText:string];
   return YES;
 }
 
 @end