[DevTools] Allow overriding only the device scale factor.

LayoutTests/inspector/device-emulation/device-emulation-test.js

 function setMetaViewport()
 {
     var VIEWPORTS = {
         "w=320": "width=320",
         "w=dw": "width=device-width",
         "w=980": "width=980",
         "none": "no viewport (desktop site)"
     };
 
     var viewport = VIEWPORTS["none"];
@@ skipping 32 matching lines @@
         return { matches: bool, media: q };
     };
 })(document);
 
 var results;
 
 function dumpMetrics()
 {
     results = [];
     writeResult("Device:", "");
+    testJS("window.screenX");
+    testJS("window.screenY");
 
     writeResult("Viewport:", location.search);
 
     testMQOrientation();
     testJS("window.orientation", "");
 
     testMQDimension("resolution", null, "dpi");
     testMQDevicePixelRatio(window.devicePixelRatio);
     testJS("window.devicePixelRatio", "");
 
@@ skipping 147 matching lines @@
     InspectorTest.emulateAndGetMetrics(width, height, deviceScaleFactor, viewport, callback);
 
     function callback()
     {
         InspectorTest.addResult(InspectorTest._deviceEmulationResults.join("\n"));
         InspectorTest.completeTest();
     }
 };
 
 };