LazyLoad: Mark automatic lazyloading as experimental

This CL marks "loading" attribute support as experimental, so that WPT
tests run with this attribute supported.

Bug: 937986
Change-Id: I797a6e467e784c1740bdd77750f7b6a3460ca58b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1700651Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: rajendrant <rajendrant@chromium.org>
Cr-Commit-Position: refs/heads/master@{#680556}

third_party/blink/renderer/core/html/parser/html_preload_scanner_test.cc

@@ -1274,6 +1274,8 @@ TEST_F(HTMLPreloadScannerTest,
   ScopedLazyImageLoadingForTest scoped_lazy_image_loading_for_test(true);
   ScopedLazyImageLoadingMetadataFetchForTest
       scoped_lazy_image_loading_metadata_fetch_for_test(true);
+  ScopedAutomaticLazyImageLoadingForTest
+      scoped_automatic_lazy_image_loading_for_test(false);
   GetDocument().GetSettings()->SetLazyLoadEnabled(true);
   RunSetUp(kViewportEnabled);
   LazyLoadImageTestCase test_cases[] = {
@@ -1333,6 +1335,8 @@ TEST_F(HTMLPreloadScannerTest,
   ScopedLazyImageLoadingForTest scoped_lazy_image_loading_for_test(true);
   ScopedLazyImageLoadingMetadataFetchForTest
       scoped_lazy_image_loading_metadata_fetch_for_test(true);
+  ScopedAutomaticLazyImageLoadingForTest
+      scoped_automatic_lazy_image_loading_for_test(false);
   GetDocument().GetSettings()->SetLazyLoadEnabled(true);
   RunSetUp(kViewportEnabled);
   PreloadScannerTestCase test_cases[] = {

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -135,10 +135,12 @@
     {
       name: "AutomaticLazyFrameLoading",
       depends_on: ["LazyFrameLoading"],
+      status: "experimental",
     },
     {
       name: "AutomaticLazyImageLoading",
       depends_on: ["LazyImageLoading"],
+      status: "experimental",
     },
     {
       name: "AutomationControlled",