Plugin Placeholders: Factor out common BindWebFrame method into base class.

components/plugins/renderer/webview_plugin.cc

Index: components/plugins/renderer/webview_plugin.cc
diff --git a/components/plugins/renderer/webview_plugin.cc b/components/plugins/renderer/webview_plugin.cc
index 6200094c08b4ac6035218dddb1c50cad9f0133df..6ecab2c8d838e17ea4de6f47f3736f062119ef5d 100644
--- a/components/plugins/renderer/webview_plugin.cc
+++ b/components/plugins/renderer/webview_plugin.cc
@@ -45,7 +45,7 @@ using content::WebPreferences;
 WebViewPlugin::WebViewPlugin(WebViewPlugin::Delegate* delegate,
                              const WebPreferences& preferences)
     : delegate_(delegate),
-      container_(NULL),
+      container_(nullptr),
       web_view_(WebView::create(this)),
       finished_loading_(false),
       focused_(false) {
@@ -61,6 +61,7 @@ WebViewPlugin* WebViewPlugin::Create(WebViewPlugin::Delegate* delegate,
                                      const WebPreferences& preferences,
                                      const std::string& html_data,
                                      const GURL& url) {
+  DCHECK(url.is_valid()) << "Blink requires the WebView to have a valid URL.";
   WebViewPlugin* plugin = new WebViewPlugin(delegate, preferences);
   plugin->web_view()->mainFrame()->loadHTMLString(html_data, url);
   return plugin;
@@ -122,9 +123,9 @@ bool WebViewPlugin::initialize(WebPluginContainer* container) {
 void WebViewPlugin::destroy() {
   if (delegate_) {
     delegate_->PluginDestroyed();
-    delegate_ = NULL;
+    delegate_ = nullptr;
   }
-  container_ = NULL;
+  container_ = nullptr;
   base::MessageLoop::current()->DeleteSoon(FROM_HERE, this);
 }
 
@@ -261,6 +262,7 @@ void WebViewPlugin::scheduleAnimation() {
 }
 
 void WebViewPlugin::didClearWindowObject(WebLocalFrame* frame) {
+  DCHECK_EQ(web_frame_, frame);
   if (delegate_)
     delegate_->BindWebFrame(frame);
 }