Retire MediaSessionSeeking runtime flag

The flag MediaSessionSeeking has been stable since 2019-08.

Bug: 977375
Change-Id: I626de5ee229586f729681825e74d3d57e8c3b36e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2209938Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Auto-Submit: Eric Willigers <ericwilligers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#772059}

third_party/blink/renderer/modules/mediasession/media_session.cc

@@ -181,13 +181,6 @@ void MediaSession::setActionHandler(const String& action,
     }
 
     UseCounter::Count(GetExecutionContext(), WebFeature::kMediaSessionSkipAd);
-  } else if (action == "seekto" &&
-             !RuntimeEnabledFeatures::MediaSessionSeekingEnabled(
-                 GetExecutionContext())) {
-    exception_state.ThrowTypeError(
-        "The provided value 'seekto' is not a valid enum "
-        "value of type MediaSessionAction.");
-    return;
   }
 
   if (handler) {

third_party/blink/renderer/modules/mediasession/media_session.idl

@@ -24,7 +24,7 @@ enum MediaSessionAction {
   "seekto"
 };
 
-callback MediaSessionActionHandler = void ([RuntimeEnabled=MediaSessionSeeking] MediaSessionActionDetails details);
+callback MediaSessionActionHandler = void (MediaSessionActionDetails details);
 
 [
     Exposed=Window,

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -1078,10 +1078,6 @@
       name: "MediaSessionPosition",
       status: "stable",
     },
-    {
-      name: "MediaSessionSeeking",
-      status: "stable",
-    },
     {
       name: "MediaSourceExperimental",
       status: "experimental",