Revert of [DevTools] Fix potential crash in RenderFrameDevToolsAgentHost...

Revert of [DevTools] Fix potential crash in RenderFrameDevToolsAgentHost (patchset #1 id:1 of https://codereview.chromium.org/1132913004/)

Reason for revert:
See crbug.com/489135

Original issue's description:
> [DevTools] Fix potential crash in RenderFrameDevToolsAgentHost
>
> BUG=483693
>
> Committed: https://crrev.com/1771428fefada2d24fab5970b915b941a5014cc1
> Cr-Commit-Position: refs/heads/master@{#330286}

TBR=dgozman@chromium.org,pfeldman@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=483693

Review URL: https://codereview.chromium.org/1140603004

Cr-Commit-Position: refs/heads/master@{#330348}

content/browser/devtools/render_frame_devtools_agent_host.cc

@@ -417,7 +417,6 @@ 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);