Upstream iOS web code.

ios/web/web_state/web_state_observer.cc

+// Copyright 2014 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 "ios/web/public/web_state/web_state_observer.h"
+
+#include "ios/web/public/load_committed_details.h"
+#include "ios/web/public/web_state/web_state.h"
+
+namespace web {
+
+WebStateObserver::WebStateObserver(WebState* web_state) : web_state_(nullptr) {
+  Observe(web_state);
+}
+
+WebStateObserver::WebStateObserver() : web_state_(nullptr) {
+}
+
+WebStateObserver::~WebStateObserver() {
+  if (web_state_)
+    web_state_->RemoveObserver(this);
+}
+
+void WebStateObserver::Observe(WebState* web_state) {
+  if (web_state == web_state_) {
+    // Early exit to avoid infinite loops if we're in the middle of a callback.
+    return;
+  }
+  if (web_state_)
+    web_state_->RemoveObserver(this);
+  web_state_ = web_state;
+  if (web_state_)
+    web_state_->AddObserver(this);
+}
+
+void WebStateObserver::ResetWebState() {
+  web_state_->RemoveObserver(this);
+  web_state_ = nullptr;
+}
+
+}  // namespace web