[DevTools] Migrate from AgentStateCallbacks to DevToolsAgentHostObserver.

content/browser/devtools/devtools_manager.cc

Index: content/browser/devtools/devtools_manager.cc
diff --git a/content/browser/devtools/devtools_manager.cc b/content/browser/devtools/devtools_manager.cc
index 70059c6f251b0ac8fe56d3ea1301d41a18db70ab..efd7bd71af292f85f485cba1ed27ea2244e1af34 100644
--- a/content/browser/devtools/devtools_manager.cc
+++ b/content/browser/devtools/devtools_manager.cc
@@ -45,34 +45,10 @@ DevToolsManager* DevToolsManager::GetInstance() {
 }
 
 DevToolsManager::DevToolsManager()
-    : delegate_(GetContentClient()->browser()->GetDevToolsManagerDelegate()),
-      attached_hosts_count_(0) {
+    : delegate_(GetContentClient()->browser()->GetDevToolsManagerDelegate()) {
 }
 
 DevToolsManager::~DevToolsManager() {
-  DCHECK(!attached_hosts_count_);
-}
-
-void DevToolsManager::AgentHostStateChanged(
-    DevToolsAgentHostImpl* agent_host, bool attached) {
-  if (attached) {
-    if (!attached_hosts_count_) {
-      BrowserThread::PostTask(
-          BrowserThread::IO,
-          FROM_HERE,
-          base::Bind(&NetLogObserver::Attach,
-                     GetContentClient()->browser()->GetNetLog()));
-    }
-    ++attached_hosts_count_;
-  } else {
-    --attached_hosts_count_;
-    if (!attached_hosts_count_) {
-      BrowserThread::PostTask(
-          BrowserThread::IO,
-          FROM_HERE,
-          base::Bind(&NetLogObserver::Detach));
-    }
-  }
 }
 
 void DevToolsManager::SetHttpHandler(