Sync NPAPI scrolling constants between files. b=477126 r/sr=jst

modules/plugin/base/public/nsplugindefs.h

@@ -357,10 +357,9 @@ enum nsPluginEventType {
     nsPluginEventType_AdjustCursorEvent,
     nsPluginEventType_MenuCommandEvent,
     nsPluginEventType_ClippingChangedEvent,
-    nsPluginEventType_ScrollingBeginsEvent,
-    nsPluginEventType_ScrollingEndsEvent,
-#endif /* XP_MACOSX */
-    nsPluginEventType_Idle                 = 0
+    nsPluginEventType_ScrollingBeginsEvent = 1000,
+    nsPluginEventType_ScrollingEndsEvent
+#endif
 };
 
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