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

ios/web/web_state/web_state_delegate.mm

Index: ios/web/web_state/web_state_delegate.mm
diff --git a/ios/web/web_state/web_state_delegate.mm b/ios/web/web_state/web_state_delegate.mm
new file mode 100644
index 0000000000000000000000000000000000000000..273f55dec9be52fa5f184d1ee8f952a5bd59c5e0
--- /dev/null
+++ b/ios/web/web_state/web_state_delegate.mm
@@ -0,0 +1,33 @@
+// 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* web_contents) {
+  DCHECK(attached_states_.find(web_contents) == attached_states_.end());
+  attached_states_.insert(web_contents);
+}
+
+void WebStateDelegate::Detach(WebState* web_contents) {
+  DCHECK(attached_states_.find(web_contents) != attached_states_.end());
+  attached_states_.erase(web_contents);
+}
+
+}  // web