[DevTools] Call InnerDetach instead of NotifyDetached in DTAHI::HostClosed.

InnerDetach destroyes the current session and domain handlers, as opposite to
NotifyDetached, which prevents stale pointers and inconsistent state at the time
of DTAHI destruction.

BUG=674474

Review-Url: https://codereview.chromium.org/2579923002
Cr-Commit-Position: refs/heads/master@{#439003}

content/browser/devtools/devtools_agent_host_impl.cc

@@ -265,7 +265,7 @@ void DevToolsAgentHostImpl::HostClosed() {
   DevToolsAgentHostClient* client = client_;
   client_ = NULL;
   client->AgentHostClosed(this, false);
-  NotifyDetached();
+  InnerDetach();
 }
 
 void DevToolsAgentHostImpl::InspectElement(int x, int y) {

content/shell/browser/shell_devtools_frontend.cc

@@ -373,6 +373,7 @@ void ShellDevToolsFrontend::SendMessageAck(int request_id,
 
 void ShellDevToolsFrontend::AgentHostClosed(
     DevToolsAgentHost* agent_host, bool replaced) {
+  agent_host_ = nullptr;
   frontend_shell_->Close();
 }