Mark LayoutShiftAttribution REF stable.

Intent to ship: bit.ly/clssesi2s

Bug: 1053510
Change-Id: Ibd24c3f11c08c71c3e83eb6f2ae1749f13170202
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2197557Reviewed-by: Nicolás Peña Moreno <npm@chromium.org>
Reviewed-by: Yoav Weiss <yoavweiss@chromium.org>
Commit-Queue: Steve Kobes <skobes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#768982}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -983,7 +983,7 @@
       // Adds the "sources" attribute to the LayoutShift interface for the
       // Layout Instability API (bit.ly/cls-sources).
       name: "LayoutShiftAttribution",
-      status: "experimental",
+      status: "stable",
     },
     {
       name: "LazyFrameLoading",

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

@@ -4085,9 +4085,17 @@ interface LayoutShift : PerformanceEntry
     attribute @@toStringTag
     getter hadRecentInput
     getter lastInputTime
+    getter sources
     getter value
     method constructor
     method toJSON
+interface LayoutShiftAttribution
+    attribute @@toStringTag
+    getter currentRect
+    getter node
+    getter previousRect
+    method constructor
+    method toJSON
 interface LinearAccelerationSensor : Accelerometer
     attribute @@toStringTag
     method constructor