LazyLoad: Mark "loading" attribute support as experimental.

This CL marks "loading" attribute support as experimental, so that tests
run with this attribute supported. Note that this only turns on lazy
loading of images and iframes marked with "loading=lazy", and does not
turn on any automatic lazy loading of images/iframes that aren't marked
"loading=lazy".

Bug: 937986
Change-Id: Iccf1c4614f648f0596198e9c76eafac818daac23
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1636639
Commit-Queue: Scott Little <sclittle@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#671465}

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

@@ -1232,6 +1232,7 @@ TEST_F(HTMLPreloadScannerTest, LazyLoadImage_DisabledForSmallImages) {
 }
 
 TEST_F(HTMLPreloadScannerTest, LazyLoadImage_FeatureDisabledWithAttribute) {
+  ScopedLazyImageLoadingForTest scoped_lazy_image_loading_for_test(false);
   GetDocument().GetSettings()->SetLazyLoadEnabled(true);
   RunSetUp(kViewportEnabled);
   LazyLoadImageTestCase test_cases[] = {

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -835,12 +835,14 @@
     },
     {
       name: "LazyFrameLoading",
+      status: "experimental",
     },
     {
       name: "LazyFrameVisibleLoadTimeMetrics",
     },
     {
       name: "LazyImageLoading",
+      status: "experimental",
     },
     {
       name: "LazyImageVisibleLoadTimeMetrics",

third_party/blink/web_tests/webexposed/element-instance-property-listing-expected.txt

@@ -638,6 +638,7 @@ html element iframe
     property frameBorder
     property getSVGDocument
     property height
+    property loading
     property longDesc
     property marginHeight
     property marginWidth
@@ -662,6 +663,7 @@ html element img
     property importance
     property intrinsicSize
     property isMap
+    property loading
     property longDesc
     property lowsrc
     property name

third_party/blink/web_tests/webexposed/global-interface-listing-expected.txt

@@ -3120,6 +3120,7 @@ interface HTMLIFrameElement : HTMLElement
     getter featurePolicy
     getter frameBorder
     getter height
+    getter loading
     getter longDesc
     getter marginHeight
     getter marginWidth
@@ -3139,6 +3140,7 @@ interface HTMLIFrameElement : HTMLElement
     setter csp
     setter frameBorder
     setter height
+    setter loading
     setter longDesc
     setter marginHeight
     setter marginWidth
@@ -3163,6 +3165,7 @@ interface HTMLImageElement : HTMLElement
     getter importance
     getter intrinsicSize
     getter isMap
+    getter loading
     getter longDesc
     getter lowsrc
     getter name
@@ -3189,6 +3192,7 @@ interface HTMLImageElement : HTMLElement
     setter importance
     setter intrinsicSize
     setter isMap
+    setter loading
     setter longDesc
     setter lowsrc
     setter name
@@ -4242,6 +4246,7 @@ interface Image
     getter importance
     getter intrinsicSize
     getter isMap
+    getter loading
     getter longDesc
     getter lowsrc
     getter name
@@ -4268,6 +4273,7 @@ interface Image
     setter importance
     setter intrinsicSize
     setter isMap
+    setter loading
     setter longDesc
     setter lowsrc
     setter name