Convert ExtensionLoaderHandler to use the new navigation callbacks.

chrome/browser/ui/webui/extensions/extension_loader_handler.h

Index: chrome/browser/ui/webui/extensions/extension_loader_handler.h
diff --git a/chrome/browser/ui/webui/extensions/extension_loader_handler.h b/chrome/browser/ui/webui/extensions/extension_loader_handler.h
index a038a0b56ef70c49527e88622ba80e324365923c..b99ad4cab4489dd9fce87398ca7c9ccb9bad8683 100644
--- a/chrome/browser/ui/webui/extensions/extension_loader_handler.h
+++ b/chrome/browser/ui/webui/extensions/extension_loader_handler.h
@@ -65,9 +65,8 @@ class ExtensionLoaderHandler : public content::WebUIMessageHandler,
                      const std::string& error) override;
 
   // content::WebContentsObserver:
-  void DidStartNavigationToPendingEntry(
-      const GURL& url,
-      content::ReloadType reload_type) override;
+  void DidStartNavigation(
+      content::NavigationHandle* navigation_handle) override;
 
   // Add a failure to |failures_|. If it was a manifest error, |manifest| will
   // hold the manifest contents, and |line_number| will point to the line at