Refactors PDFHistory.

# Conflicts:
#	extensions/b2g/viewer.js
#	web/pdf_history.js
#	web/pdf_viewer.component.js
#	web/viewer.js

web/interfaces.js

@@ -59,6 +59,17 @@ IPDFLinkService.prototype = {
   cachePageRef: function (pageNum, pageRef) {},
 };
 
+/**
+ * @interface
+ */
+function IPDFHistory() {}
+IPDFHistory.prototype = {
+  forward: function () {},
+  back: function () {},
+  push: function (params) {},
+  updateNextHashParam: function (hash) {},
+};
+
 /**
  * @interface
  */