remove the useless NPAPI struct

NPAPI is not used in WebPlugin.h, remove it

BUG=none
R=haraken@chromium.org

Review-Url: https://codereview.chromium.org/2481213002
Cr-Commit-Position: refs/heads/master@{#430287}

third_party/WebKit/public/web/WebPlugin.h

@@ -41,9 +41,6 @@
 #include "WebWidget.h"
 #include <v8.h>
 
-struct NPObject;
-struct _NPP;
-
 namespace blink {
 
 class WebDragData;