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

ios/web/web_state/web_state_delegate_bridge.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_bridge.h"
+
+#include "base/logging.h"
+
+namespace web {
+
+WebStateDelegateBridge::WebStateDelegateBridge(id<CRWWebStateDelegate> delegate)
+    : delegate_(delegate) {}
+
+WebStateDelegateBridge::~WebStateDelegateBridge() {}
+
+void WebStateDelegateBridge::LoadProgressChanged(WebState* source,
+                                                 double progress) {
+  if ([delegate_ respondsToSelector:@selector(webState:didChangeProgress:)])
+    [delegate_ webState:source didChangeProgress:progress];
+}
+
+}  // web