Make SpellCheckPanel compiled and used only on Mac

components/spellcheck/renderer/spellcheck_panel.h

 // Copyright 2017 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 COMPONENTS_SPELLCHECK_RENDERER_SPELLCHECK_PANEL_H
 #define COMPONENTS_SPELLCHECK_RENDERER_SPELLCHECK_PANEL_H
 
 #include "base/macros.h"
 #include "components/spellcheck/spellcheck_build_features.h"
 #include "content/public/renderer/render_view_observer.h"
 #include "content/public/renderer/render_view_observer_tracker.h"
 #include "third_party/WebKit/public/web/WebSpellCheckClient.h"
 
-// TODO(xiaochengh): Only Mac has spelling panel. Remove the #if checks in this
-// class, which seem unnecessary, and make sure that this class is compiled and
-// used only on Mac.
+#if !BUILDFLAG(HAS_SPELLCHECK_PANEL)
+#error "This file shouldn't be compiled without spellcheck panel."
+#endif
+
 class SpellCheckPanel
     : public content::RenderViewObserver,
       public content::RenderViewObserverTracker<SpellCheckPanel>,
       public blink::WebSpellCheckClient {
  public:
   explicit SpellCheckPanel(content::RenderView* render_view);
   ~SpellCheckPanel() override;
 
   // RenderViewObserver implementation.
   bool OnMessageReceived(const IPC::Message& message) override;
 
  private:
   // RenderViewObserver implementation.
   void OnDestruct() override;
 
   // blink::WebSpellCheckClient implementation.
   void ShowSpellingUI(bool show) override;
   bool IsShowingSpellingUI() override;
   void UpdateSpellingUIWithMisspelledWord(
       const blink::WebString& word) override;
 
-#if BUILDFLAG(USE_BROWSER_SPELLCHECKER)
   void OnAdvanceToNextMisspelling();
   void OnToggleSpellPanel(bool is_currently_visible);
-#endif
 
   // True if the browser is showing the spelling panel for us.
   bool spelling_panel_visible_;
 
   DISALLOW_COPY_AND_ASSIGN(SpellCheckPanel);
 };
 
 #endif