Use srcset's resource pixel density to determine intrinsic size

LayoutTests/fast/hidpi/resources/srcset-helper.js

 function runTest() {
     if (!window.testRunner || !window.sessionStorage)
         return;
 
     if (!window.targetScaleFactor)
         window.targetScaleFactor = 2;
 
     if (!sessionStorage.scaleFactorIsSet) {
         testRunner.waitUntilDone();
         testRunner.setBackingScaleFactor(targetScaleFactor, scaleFactorIsSet);
     }
 
     if (sessionStorage.pageReloaded && sessionStorage.scaleFactorIsSet) {
         delete sessionStorage.pageReloaded;
         delete sessionStorage.scaleFactorIsSet;
-        testRunner.notifyDone();
+        if (!window.manualNotifyDone)
+            testRunner.notifyDone();
     } else {
         // Right now there is a bug that srcset does not properly deal with dynamic changes to the scale factor,
         // so to work around that, we must reload the page to get the new image.
         sessionStorage.pageReloaded = true;
         document.location.reload(true);
     }
 }
 
 function scaleFactorIsSet() {
     sessionStorage.scaleFactorIsSet = true;
 }
 
 window.addEventListener("load", runTest, false);