Async Clipboard: Remove experimental flag from navigator.clipboard.{read,write}

Enables the Async Clipboard API's read and write functions, with ClipboardItem,
on stable, by updating runtime_enabled_features flags from experimental to stable.

Spec: https://w3c.github.io/clipboard-apis/#async-clipboard-api
Intent to Ship: https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/TssvWe8TzBc

Bug: 150835
Change-Id: I602defa5f300832c2322e3dccb444b4c807311ef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1567224Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Commit-Queue: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#665227}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -117,7 +117,7 @@
     },
     {
       name: "AsyncClipboard",
-      status: "experimental",
+      status: "stable",
     },
     {
       name: "AudioOutputDevices",

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

@@ -845,12 +845,19 @@ interface CharacterData : Node
 interface Clipboard : EventTarget
     attribute @@toStringTag
     method constructor
+    method read
     method readText
+    method write
     method writeText
 interface ClipboardEvent : Event
     attribute @@toStringTag
     getter clipboardData
     method constructor
+interface ClipboardItem
+    attribute @@toStringTag
+    getter types
+    method constructor
+    method getType
 interface CloseEvent : Event
     attribute @@toStringTag
     getter code