Bug 1570490. Remove some unnecessary argument name escapes. r=mccr8

Differential Revision: https://phabricator.services.mozilla.com/D40109

--HG--
extra : moz-landing-system : lando

dom/webidl/DataTransferItem.webidl

@@ -12,7 +12,7 @@ interface DataTransferItem {
   readonly attribute DOMString kind;
   readonly attribute DOMString type;
   [Throws, NeedsSubjectPrincipal]
-  void getAsString(FunctionStringCallback? _callback);
+  void getAsString(FunctionStringCallback? callback);
   [Throws, NeedsSubjectPrincipal]
   File? getAsFile();
 };

dom/webidl/HTMLCanvasElement.webidl

@@ -27,7 +27,7 @@ interface HTMLCanvasElement : HTMLElement {
   DOMString toDataURL(optional DOMString type = "",
                       optional any encoderOptions);
   [Throws, NeedsSubjectPrincipal]
-  void toBlob(BlobCallback _callback,
+  void toBlob(BlobCallback callback,
               optional DOMString type = "",
               optional any encoderOptions);
 };