Fix a crash related to null BrowserPluginGuest on Mac

content/browser/frame_host/render_widget_host_view_guest.cc

 // 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.
 
 #include "content/browser/frame_host/render_widget_host_view_guest.h"
 
 #include <utility>
 
 #include "base/bind_helpers.h"
 #include "base/command_line.h"
@@ skipping 526 matching lines @@
 void RenderWidgetHostViewGuest::LockCompositingSurface() {
   NOTIMPLEMENTED();
 }
 
 void RenderWidgetHostViewGuest::UnlockCompositingSurface() {
   NOTIMPLEMENTED();
 }
 
 RenderWidgetHostViewBase*
 RenderWidgetHostViewGuest::GetOwnerRenderWidgetHostView() const {
-  return static_cast<RenderWidgetHostViewBase*>(
-      guest_->GetOwnerRenderWidgetHostView());
+  return guest_ ? static_cast<RenderWidgetHostViewBase*>(
+                      guest_->GetOwnerRenderWidgetHostView())
+                : nullptr;
 }
 
 // TODO(wjmaclean): When we remove BrowserPlugin, delete this code.
 // http://crbug.com/533069
 void RenderWidgetHostViewGuest::MaybeSendSyntheticTapGesture(
     const blink::WebFloatPoint& position,
     const blink::WebFloatPoint& screenPosition) const {
   if (!HasFocus()) {
     // We need to a account for the position of the guest view within the
     // embedder, as well as the fact that the embedder's host will add its
@@ skipping 130 matching lines @@
         gesture_event.data.scrollUpdate.inertialPhase ==
             blink::WebGestureEvent::MomentumPhase) {
       return;
     }
     host_->ForwardGestureEvent(gesture_event);
     return;
   }
 }
 
 }  // namespace content