[DevTools] Do not allow DTAH to attach a single client twice

Attaching second time destroys the first session (by overriding it
in session_by_client_ map), and leaves stale pointers to it in
session_by_id_ and sessions_.

Bug: 774695
Change-Id: Ibb2dad0917adaf1c33bace4feef8e4fc5247f7c8
Reviewed-on: https://chromium-review.googlesource.com/719431Reviewed-by: Pavel Feldman <pfeldman@chromium.org>
Commit-Queue: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#509245}

content/browser/devtools/devtools_agent_host_impl.cc

@@ -157,10 +157,14 @@ void DevToolsAgentHostImpl::InnerAttachClient(DevToolsAgentHostClient* client) {
 }
 
 void DevToolsAgentHostImpl::AttachClient(DevToolsAgentHostClient* client) {
+  if (SessionByClient(client))
+    return;
   InnerAttachClient(client);
 }
 
 void DevToolsAgentHostImpl::ForceAttachClient(DevToolsAgentHostClient* client) {
+  if (SessionByClient(client))
+    return;
   scoped_refptr<DevToolsAgentHostImpl> protect(this);
   if (!sessions_.empty())
     ForceDetachAllClients(true);