[Media Router] Clean up issues related code.

chrome/browser/media/router/issues_observer.cc

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/media/router/issues_observer.h"
 
 #include "base/logging.h"
 #include "chrome/browser/media/router/media_router.h"
 
 namespace media_router {
 
-IssuesObserver::IssuesObserver(MediaRouter* router) : router_(router) {
+IssuesObserver::IssuesObserver(MediaRouter* router)
+    : router_(router), initialized_(false) {
   DCHECK(router_);
 }
 
 IssuesObserver::~IssuesObserver() {
+  if (initialized_)
+    router_->UnregisterIssuesObserver(this);
 }
 
-void IssuesObserver::RegisterObserver() {
+void IssuesObserver::Init() {
+  if (initialized_)
+    return;
+
   router_->RegisterIssuesObserver(this);
-}
-
-void IssuesObserver::UnregisterObserver() {
-  router_->UnregisterIssuesObserver(this);
+  initialized_ = true;

[dcheng, 2016/12/07 18:58:02]

Can you help me understand why this isn't just part of the constructor? Doing so
would remove the need for |initialized_| completely.

[imcheng, 2016/12/09 23:19:53]

RegisterIssuesObserver will synchronously invoke OnIssue() if there is currently
an issue, which is a virtual method -- this results in undefined behavior. Also
doing work in constructors is discouraged by style guideline.

 }
 
 }  // namespace media_router