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

ios/web/web_state/ui/crw_wk_web_view_web_controller.mm

 // 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.
 
 #import "ios/web/web_state/ui/crw_wk_web_view_web_controller.h"
 
 #include <stddef.h>
 
 #include <utility>
 
@@ skipping 1169 matching lines @@
                       ofObject:(id)object
                         change:(NSDictionary*)change
                        context:(void*)context {
   NSString* dispatcherSelectorName = self.wkWebViewObservers[keyPath];
   DCHECK(dispatcherSelectorName);
   if (dispatcherSelectorName)
     [self performSelector:NSSelectorFromString(dispatcherSelectorName)];
 }
 
 - (void)webViewEstimatedProgressDidChange {
+  if ([self isBeingDestroyed])
+    return;
+
+  self.webStateImpl->SendChangeLoadProgress([_wkWebView estimatedProgress]);
   if ([self.delegate respondsToSelector:
           @selector(webController:didUpdateProgress:)]) {
     [self.delegate webController:self
                didUpdateProgress:[_wkWebView estimatedProgress]];
   }
 }
 
 - (void)webViewSecurityFeaturesDidChange {
   if (self.loadPhase == web::LOAD_REQUESTED) {
     // Do not update SSL Status for pending load. It will be updated in
@@ skipping 522 matching lines @@
         runJavaScriptTextInputPanelWithPrompt:prompt
                                   defaultText:defaultText
                                    requestURL:requestURL
                             completionHandler:completionHandler];
   } else if (completionHandler) {
     completionHandler(nil);
   }
 }
 
 @end