Add preloader support for picture based source selection

LayoutTests/http/tests/loading/resources/picture-preload-helper.js

Index: LayoutTests/http/tests/loading/resources/picture-preload-helper.js
diff --git a/LayoutTests/http/tests/loading/resources/picture-preload-helper.js b/LayoutTests/http/tests/loading/resources/picture-preload-helper.js
new file mode 100644
index 0000000000000000000000000000000000000000..11e41dbd8cc1caf4b10a9945f699359483d8e159
--- /dev/null
+++ b/LayoutTests/http/tests/loading/resources/picture-preload-helper.js
@@ -0,0 +1,20 @@
+function loadFromSource(i) {
+    shouldBeFalse("internals.isPreloaded('resources/preload-test.jpg?" + i + "');");
+    shouldBeFalse("internals.isPreloaded('resources/base-image1.png?" + i + "');");
+    shouldBeFalse("internals.isPreloaded('resources/base-image2.png?" + i + "');");
+    shouldBeTrue("internals.isPreloaded('resources/base-image3.png?" + i + "');");
+}
+
+function loadFromSource2x(i) {
+    shouldBeFalse("internals.isPreloaded('resources/preload-test.jpg?" + i + "');");
+    shouldBeFalse("internals.isPreloaded('resources/base-image1.png?" + i + "');");
+    shouldBeTrue("internals.isPreloaded('resources/base-image2.png?" + i + "');");
+    shouldBeFalse("internals.isPreloaded('resources/base-image3.png?" + i + "');");
+}
+
+function loadFromImg(i) {
+    shouldBeTrue("internals.isPreloaded('resources/preload-test.jpg?" + i + "');");
+    shouldBeFalse("internals.isPreloaded('resources/base-image1.png?" + i + "');");
+    shouldBeFalse("internals.isPreloaded('resources/base-image2.png?" + i + "');");
+    shouldBeFalse("internals.isPreloaded('resources/base-image3.png?" + i + "');");
+}