Remove redundant DevToolsManager methods and clean up its clients.

chrome/browser/extensions/api/debugger/debugger_api.cc

Index: chrome/browser/extensions/api/debugger/debugger_api.cc
diff --git a/chrome/browser/extensions/api/debugger/debugger_api.cc b/chrome/browser/extensions/api/debugger/debugger_api.cc
index 0af2f985022f70c46a6fe87158641aff2101903c..ade151aebb4bd9b1191396fa79051260e738cf51 100644
--- a/chrome/browser/extensions/api/debugger/debugger_api.cc
+++ b/chrome/browser/extensions/api/debugger/debugger_api.cc
@@ -525,10 +525,7 @@ bool DebuggerAttachFunction::RunImpl() {
 
   scoped_refptr<DevToolsAgentHost> agent(DevToolsAgentHost::GetOrCreateFor(
       contents_->GetRenderViewHost()));
-  DevToolsClientHost* client_host = DevToolsManager::GetInstance()->
-      GetDevToolsClientHostFor(agent);
-
-  if (client_host != NULL) {
+  if (agent->IsAttached()) {
     FormatErrorMessage(keys::kAlreadyAttachedError);
     return false;
   }