Add window.screenLeft and window.screenTop as aliases

Given that usage of these attributes are high and that they are implemented in Chrome, WebKit, and Edge, standardization seems like the best option for screenLeft and screenTop. Add them into the spec as simple aliases of screenX and screenY.

Fixes w3c#1091.

Author: csnardi

cssom-view-1/Overview.bs

@@ -471,7 +471,9 @@ partial interface Window {
 
     // client
     [Replaceable] readonly attribute long screenX;
+    [Replaceable] readonly attribute long screenLeft;
     [Replaceable] readonly attribute long screenY;
+    [Replaceable] readonly attribute long screenTop;
     [Replaceable] readonly attribute long outerWidth;
     [Replaceable] readonly attribute long outerHeight;
     [Replaceable] readonly attribute double devicePixelRatio;
@@ -626,12 +628,12 @@ user agent must run these steps:
 1. Add the value of {{scrollY}} to the {{ScrollToOptions/top}} dictionary member.
 1. Act as if the {{Window/scroll()}} method was invoked with <var>options</var> as the only argument.
 
-The <dfn attribute for=Window>screenX</dfn> attribute must return the x-coordinate,
+The <dfn attribute for=Window>screenX</dfn> and <dfn attribute for=Window>screenLeft</dfn> attributes must return the x-coordinate,
 relative to the origin of the <a>Web-exposed screen area</a>, of the left of
 the client window as number of <a lt=px value>CSS pixels</a>, or zero if there is no such
 thing. <!--fingerprint-->
 
-The <dfn attribute for=Window>screenY</dfn> attribute must return the y-coordinate,
+The <dfn attribute for=Window>screenY</dfn> and <dfn attribute for=Window>screenTop</dfn> attributes must return the y-coordinate,
 relative to the origin of the screen of the <a>Web-exposed screen area</a>, of the top of
 the client window as number of <a lt=px value>CSS pixels</a>, or zero if there is no such
 thing. <!--fingerprint-->