[worklets] Change paintWorklet method to non-const to remove mutable member.

third_party/WebKit/Source/modules/csspaint/WindowPaintWorklet.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef WindowPaintWorklet_h
 #define WindowPaintWorklet_h
 
 #include "core/frame/DOMWindowProperty.h"
 #include "modules/ModulesExport.h"
 #include "platform/Supplementable.h"
 #include "platform/heap/Handle.h"
 
 namespace blink {
 
 class DOMWindow;
 class ExecutionContext;
 class PaintWorklet;
 class Worklet;
 
 class MODULES_EXPORT WindowPaintWorklet final : public GarbageCollected<WindowPaintWorklet>, public Supplement<LocalDOMWindow>, public DOMWindowProperty {
     USING_GARBAGE_COLLECTED_MIXIN(WindowPaintWorklet);
 public:
     static WindowPaintWorklet& from(LocalDOMWindow&);
     static Worklet* paintWorklet(ExecutionContext*, DOMWindow&);
-    PaintWorklet* paintWorklet(ExecutionContext*) const;
+    PaintWorklet* paintWorklet(ExecutionContext*);
 
     DECLARE_TRACE();
 
 private:
     explicit WindowPaintWorklet(LocalDOMWindow&);
     static const char* supplementName();
 
-    mutable Member<PaintWorklet> m_paintWorklet;
+    Member<PaintWorklet> m_paintWorklet;
 };
 
 } // namespace blink
 
 #endif // WindowPaintWorklet_h