[DevTools] Implement DevToolsManager::Observer which notifies about target updates.

content/browser/devtools/devtools_agent_host_impl.cc

Index: content/browser/devtools/devtools_agent_host_impl.cc
diff --git a/content/browser/devtools/devtools_agent_host_impl.cc b/content/browser/devtools/devtools_agent_host_impl.cc
index 0630a0473be002c67659a9a17d0bf043922f8410..b7f9b24b59f5a749f37691ebb4ed0d40878b3a6a 100644
--- a/content/browser/devtools/devtools_agent_host_impl.cc
+++ b/content/browser/devtools/devtools_agent_host_impl.cc
@@ -75,11 +75,10 @@ void DevToolsAgentHostImpl::AttachClient(DevToolsAgentHostClient* client) {
   if (client_) {
     client_->AgentHostClosed(this, true);
     Detach();
-  } else {
-    DevToolsManager::GetInstance()->OnClientAttached();
   }
   client_ = client;
   Attach();
+  DevToolsManager::GetInstance()->AgentHostChanged(this);
 }
 
 void DevToolsAgentHostImpl::DetachClient() {
@@ -88,8 +87,8 @@ void DevToolsAgentHostImpl::DetachClient() {
 
   scoped_refptr<DevToolsAgentHostImpl> protect(this);
   client_ = NULL;
-  DevToolsManager::GetInstance()->OnClientDetached();
   Detach();
+  DevToolsManager::GetInstance()->AgentHostChanged(this);
 }
 
 bool DevToolsAgentHostImpl::IsAttached() {
@@ -125,8 +124,8 @@ void DevToolsAgentHostImpl::HostClosed() {
   // Clear |client_| before notifying it.
   DevToolsAgentHostClient* client = client_;
   client_ = NULL;
-  DevToolsManager::GetInstance()->OnClientDetached();
   client->AgentHostClosed(this, false);
+  DevToolsManager::GetInstance()->AgentHostChanged(this);
 }
 
 void DevToolsAgentHostImpl::SendMessageToClient(const std::string& message) {
@@ -150,9 +149,9 @@ void DevToolsAgentHost::DetachAllClients() {
       // Clear |client_| before notifying it.
       DevToolsAgentHostClient* client = agent_host->client_;
       agent_host->client_ = NULL;
-      DevToolsManager::GetInstance()->OnClientDetached();
       client->AgentHostClosed(agent_host, true);
       agent_host->Detach();
+      DevToolsManager::GetInstance()->AgentHostChanged(protect);
     }
   }
 }