Prevent browser thumbnailer from trying to read from frontbuffer (surface texture) when none exists.

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/bind_helpers.h"
@@ skipping 503 matching lines @@
 
 void RenderWidgetHostViewMac::Blur() {
   UnlockMouse();
   [[cocoa_view_ window] makeFirstResponder:nil];
 }
 
 bool RenderWidgetHostViewMac::HasFocus() const {
   return [[cocoa_view_ window] firstResponder] == cocoa_view_;
 }
 
+bool RenderWidgetHostViewMac::IsSurfaceAvailableForCopy() const {
+  return !!render_widget_host_->GetBackingStore(false) ||
+      (compositing_iosurface_.get() && compositing_iosurface_->HasIOSurface());
+}
+
 void RenderWidgetHostViewMac::Show() {
   [cocoa_view_ setHidden:NO];
 
   DidBecomeSelected();
 }
 
 void RenderWidgetHostViewMac::Hide() {
   [cocoa_view_ setHidden:YES];
 
   WasHidden();
@@ skipping 2394 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