Enable srcset support in HTMLImageElement

LayoutTests/http/tests/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();
+    } 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);