[DevTools] Fix potential crash in RenderFrameDevToolsAgentHost

content/browser/devtools/render_frame_devtools_agent_host.cc

 // 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/devtools/render_frame_devtools_agent_host.h"
 
 #include "base/basictypes.h"
 #include "base/lazy_instance.h"
 #include "base/strings/utf_string_conversions.h"
 #include "content/browser/child_process_security_policy_impl.h"
@@ skipping 399 matching lines @@
 
   if (emulation_handler_)
     emulation_handler_->SetRenderFrameHost(render_frame_host_);
   if (page_handler_)
     page_handler_->SetRenderFrameHost(render_frame_host_);
 }
 
 void RenderFrameDevToolsAgentHost::ClearRenderFrameHost() {
   DCHECK(render_frame_host_);
   render_frame_host_ = nullptr;
+  WebContentsObserver::Observe(nullptr);
   dom_handler_->SetRenderFrameHost(nullptr);
   if (emulation_handler_)
     emulation_handler_->SetRenderFrameHost(nullptr);
   input_handler_->SetRenderWidgetHost(nullptr);
   network_handler_->SetRenderFrameHost(nullptr);
   if (page_handler_)
     page_handler_->SetRenderFrameHost(nullptr);
   service_worker_handler_->SetRenderFrameHost(nullptr);
 }
 
@@ skipping 96 matching lines @@
   if (!IsAttached() || !render_frame_host_)
     return;
   ProcessChunkedMessageFromAgent(message);
 }
 
 bool RenderFrameDevToolsAgentHost::IsChildFrame() {
   return render_frame_host_ && render_frame_host_->GetParent();
 }
 
 }  // namespace content