[DevTools] Fix potential crash in RenderFrameDevToolsAgentHost

BUG=483693

Review URL: https://codereview.chromium.org/1132913004

Cr-Commit-Position: refs/heads/master@{#330286}

content/browser/devtools/render_frame_devtools_agent_host.cc

@@ -417,6 +417,7 @@ void RenderFrameDevToolsAgentHost::SetRenderFrameHost(RenderFrameHost* rfh) {
 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);