[ios] Added web// public API to let embedder observe loading progress.

ios/web/web_state/web_state_delegate.mm

+// Copyright 2016 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.
+
+#import "ios/web/public/web_state/web_state_delegate.h"
+
+#import "ios/web/public/web_state/web_state.h"
+
+namespace web {
+
+WebStateDelegate::WebStateDelegate() {}
+
+WebStateDelegate::~WebStateDelegate() {
+  while (!attached_states_.empty()) {
+    WebState* web_state = *attached_states_.begin();
+    web_state->SetDelegate(nullptr);
+  }
+  DCHECK(attached_states_.empty());
+}
+
+void WebStateDelegate::LoadProgressChanged(WebState*, double) {}
+
+void WebStateDelegate::Attach(WebState* source) {
+  DCHECK(attached_states_.find(source) == attached_states_.end());
+  attached_states_.insert(source);
+}
+
+void WebStateDelegate::Detach(WebState* source) {
+  DCHECK(attached_states_.find(source) != attached_states_.end());
+  attached_states_.erase(source);
+}
+
+}  // web