Remove visibilityChangeOnUnload runtime flag (as it's shipped)

Feature has shipped in M56 and two milestones have passed, we should
be able to remove the runtime flag now.

BUG=554834

Review-Url: https://codereview.chromium.org/2895543004
Cr-Commit-Position: refs/heads/master@{#473447}

third_party/WebKit/Source/core/dom/Document.cpp

@@ -3244,8 +3244,7 @@ void Document::DispatchUnloadEvents() {
 
       PageVisibilityState visibility_state = GetPageVisibilityState();
       load_event_progress_ = kUnloadVisibilityChangeInProgress;
-      if (visibility_state != kPageVisibilityStateHidden &&
-          RuntimeEnabledFeatures::visibilityChangeOnUnloadEnabled()) {
+      if (visibility_state != kPageVisibilityStateHidden) {
         // Dispatch visibilitychange event, but don't bother doing
         // other notifications as we're about to be unloaded.
         DispatchEvent(Event::CreateBubble(EventTypeNames::visibilitychange));

third_party/WebKit/Source/platform/RuntimeEnabledFeatures.json5

@@ -987,10 +987,6 @@
     {
       name: "VideoRotateToFullscreen",
     },
-    {
-      name: "VisibilityChangeOnUnload",
-      status: "stable",
-    },
     {
       name: "VisualViewportAPI",
       status: "experimental",