Enable srcset support in HTMLImageElement

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

Index: LayoutTests/fast/hidpi/resources/srcset-helper.js
diff --git a/LayoutTests/fast/hidpi/resources/srcset-helper.js b/LayoutTests/fast/hidpi/resources/srcset-helper.js
new file mode 100644
index 0000000000000000000000000000000000000000..703b266e967682203b850d9759ac3870c624b9db
--- /dev/null
+++ b/LayoutTests/fast/hidpi/resources/srcset-helper.js
@@ -0,0 +1,29 @@
+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);