[DevTools] Migrate from AgentStateCallbacks to DevToolsAgentHostObserver.

content/public/browser/devtools_manager_delegate.cc

Index: content/public/browser/devtools_manager_delegate.cc
diff --git a/content/public/browser/devtools_manager_delegate.cc b/content/public/browser/devtools_manager_delegate.cc
index 3e314535ecb4365871c5e4fd43722d0da8f0732d..b4d12ed947a1b8851baba0c75c5accdf20e1586e 100644
--- a/content/public/browser/devtools_manager_delegate.cc
+++ b/content/public/browser/devtools_manager_delegate.cc
@@ -10,10 +10,6 @@ namespace content {
 void DevToolsManagerDelegate::Inspect(DevToolsAgentHost* agent_host) {
 }
 
-void DevToolsManagerDelegate::DevToolsAgentStateChanged(
-    DevToolsAgentHost* agent_host, bool attached) {
-}
-
 std::string DevToolsManagerDelegate::GetTargetType(RenderFrameHost* host) {
   return std::string();
 }