[ElementTiming] Ship ElementTiming API

Intent to Ship:
https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/DXSoS7dMmok

Bug: 879270, 942033, 883483, 972830
Change-Id: Ie6255ed53ec00a083af37e71ebae34e48f72c3f2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1713275
Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
Reviewed-by: Yoav Weiss <yoavweiss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#680160}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -513,7 +513,7 @@
       // https://crbug.com/879270
       name: "ElementTiming",
       origin_trial_feature_name: "ElementTimingImages",
-      status: "experimental",
+      status: "stable",
     },
     {
       name: "EncryptedMediaEncryptionSchemeQuery",

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

@@ -4657,6 +4657,19 @@ interface Performance : EventTarget
     method setResourceTimingBufferSize
     method toJSON
     setter onresourcetimingbufferfull
+interface PerformanceElementTiming : PerformanceEntry
+    attribute @@toStringTag
+    getter element
+    getter id
+    getter identifier
+    getter intersectionRect
+    getter loadTime
+    getter naturalHeight
+    getter naturalWidth
+    getter renderTime
+    getter url
+    method constructor
+    method toJSON
 interface PerformanceEntry
     attribute @@toStringTag
     getter duration