Retire ResizeObserver runtime flag

ResizeObserver shipped to stable in M64
https://chromium-review.googlesource.com/754562
https://www.chromestatus.com/features/5705346022637568

BUG=612962

Change-Id: I8b323722c3bf8caa028ec5d14964b43dc13d1faa
Reviewed-on: https://chromium-review.googlesource.com/1163060Reviewed-by: Aleks Totic <atotic@chromium.org>
Reviewed-by: Yoav Weiss <yoav@yoav.ws>
Commit-Queue: Eric Willigers <ericwilligers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581054}

third_party/blink/renderer/core/resize_observer/resize_observer.idl

@@ -11,8 +11,7 @@ callback ResizeObserverCallback = void (sequence<ResizeObserverEntry> entries, R
 [
     ActiveScriptWrappable,
     Constructor(ResizeObserverCallback callback),
-    ConstructorCallWith=Document,
-    RuntimeEnabled=ResizeObserver
+    ConstructorCallWith=Document
 ] interface ResizeObserver {
     void observe(Element target);
     void unobserve(Element target);

third_party/blink/renderer/core/resize_observer/resize_observer_entry.idl

@@ -4,9 +4,7 @@
 
 // https://wicg.github.io/ResizeObserver/#resize-observer-entry-interface
 
-[
-    RuntimeEnabled=ResizeObserver
-] interface ResizeObserverEntry {
+interface ResizeObserverEntry {
     readonly attribute Element target;
     readonly attribute DOMRectReadOnly contentRect;
 };

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -1086,10 +1086,6 @@
       name: "RequireCSSExtensionForFile",
       status: "stable",
     },
-    {
-      name: "ResizeObserver",
-      status: "stable",
-    },
     {
       name: "ResourceLoadScheduler",
       status: "experimental",