Revert IDL for DOMPoint constructor; x and y should be required to disambiguate. Fixes https://www.w3.org/Bugs/Public/show_bug.cgi?id=28108

Author: zcorpan

geometry/Overview.bs

@@ -57,7 +57,7 @@ interface DOMPointReadOnly {
 };
 
 [Constructor(optional DOMPointInit point),
- Constructor(optional unrestricted double x = 0, optional unrestricted double y = 0,
+ Constructor(unrestricted double x, unrestricted double y,
              optional unrestricted double z = 0, optional unrestricted double w = 1),
  Exposed=(Window,Worker)]
 interface DOMPoint : DOMPointReadOnly {