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

content/child/npapi/plugin_string_stream.h

Index: content/child/npapi/plugin_string_stream.h
diff --git a/content/child/npapi/plugin_string_stream.h b/content/child/npapi/plugin_string_stream.h
index 73dac4418983b910f80a75de8851e57cfda66278..7d09c044653c501810ee612df80d8f07c239452e 100644
--- a/content/child/npapi/plugin_string_stream.h
+++ b/content/child/npapi/plugin_string_stream.h
@@ -20,9 +20,7 @@ class PluginStringStream : public PluginStream {
   // If notify_needed, will notify the plugin after the data has
   // all been sent.
   PluginStringStream(PluginInstance* instance,
-                     const GURL& url,
-                     bool notify_needed,
-                     void* notify_data);
+                     const GURL& url);
 
   // Initiates the sending of data to the plugin.
   void SendToPlugin(const std::string& data,