Remove PluginLoadObserver and related logic, it was only used for NPAPI.

content/child/npapi/webplugin.h

Index: content/child/npapi/webplugin.h
diff --git a/content/child/npapi/webplugin.h b/content/child/npapi/webplugin.h
index c1281f14d2464317b9a53068640fbbdc9e179213..2bd2b0ea56b2f0f24405791cb25d546225f218f7 100644
--- a/content/child/npapi/webplugin.h
+++ b/content/child/npapi/webplugin.h
@@ -71,14 +71,12 @@ class WebPlugin {
                                  const GURL& first_party_for_cookies) = 0;
 
   // Handles GetURL/GetURLNotify/PostURL/PostURLNotify requests initiated
-  // by plugins.  If the plugin wants notification of the result, notify_id will
-  // be non-zero.
+  // by plugins.
   virtual void HandleURLRequest(const char* url,
                                 const char* method,
                                 const char* target,
                                 const char* buf,
                                 unsigned int len,
-                                int notify_id,
                                 bool popups_allowed,
                                 bool notify_redirects) = 0;