[LargestContentfulPaint] Ship LargestContentfulPaint API

Intent to Ship:
https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/S36p6Y16u5c

Bug: 965505, 972038
Change-Id: Id2aac424b508ea698da5c73972d4c1b0442a2618
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1715739
Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
Auto-Submit: Nicolás Peña Moreno <npm@chromium.org>
Reviewed-by: Rick Byers <rbyers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#681116}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -803,7 +803,7 @@
     },
     {
       name: "LargestContentfulPaint",
-      status: "experimental",
+      status: "stable",
     },
     {
       // Exposes layout shift scores to Javascript. See explainer:

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

@@ -3787,6 +3787,16 @@ interface KeyframeEffect : AnimationEffect
     getter target
     method constructor
     setter target
+interface LargestContentfulPaint : PerformanceEntry
+    attribute @@toStringTag
+    getter element
+    getter id
+    getter loadTime
+    getter renderTime
+    getter size
+    getter url
+    method constructor
+    method toJSON
 interface LayoutShift : PerformanceEntry
     attribute @@toStringTag
     getter hadRecentInput