[DevTools] Fix potential crash in RenderFrameDevToolsAgentHost

content/browser/devtools/render_frame_devtools_agent_host.cc

Index: content/browser/devtools/render_frame_devtools_agent_host.cc
diff --git a/content/browser/devtools/render_frame_devtools_agent_host.cc b/content/browser/devtools/render_frame_devtools_agent_host.cc
index 6f40d6fae6aa8b86237eeb9a44f1ed6f5efb87f9..665885a2f96cd33c78765f585e5dce334148d1d0 100644
--- a/content/browser/devtools/render_frame_devtools_agent_host.cc
+++ b/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);