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### Preflight Checklist
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* [x] I have searched the issue tracker for an issue that matches the one I want to file, without success.

### Issue Details

* **Electron Version:**
  * <!-- (output of `node_modules/.bin/electron --version`) e.g. 4.0.3 --> 4.1.4
* **Operating System:** 
  * <!-- (Platform and Version) e.g. macOS 10.13.6 / Windows 10 (1803) / Ubuntu 18.04 x64 -->macOS
* **Last Known Working Electron version:**:
  * <!-- (if applicable) e.g. 3.1.0 --> Never

### Expected Behavior
When `titleBarStyle` is set to `customButtonsOnHover`, the semaphore buttons *should not* appear on hover if the `maximizable` and `minimizable` options are set to `false`.

### Actual Behavior
When `titleBarStyle` is set to `customButtonsOnHover`, the semaphore buttons appear on hover regardless if the `maximizable` and `minimizable` options are set to `false`.

### To Reproduce
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### Screenshots
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### Additional Information
The reason for this issue is to be able to get rid of the gray line at the top of the `BrowserWindow` described here https://github.com/electron/electron/issues/18364

atom/browser/native_window_mac.h
void SetVisibleOnAllWorkspaces(bool visible,
                                 bool visibleOnFullScreen) override;
  bool IsVisibleOnAllWorkspaces() override;
  void SetAutoHideCursor(bool auto_hide) override;
  void SelectPreviousTab() override;
  void SelectNextTab() override;
  void MergeAllWindows() override;
  void MoveTabToNewWindow() override;
  void ToggleTabBar() override;
  bool AddTabbedWindow(NativeWindow* window) override;
  bool SetWindowButtonVisibility(bool visible) override;
  void SetVibrancy(const std::string& type) override;
  void SetTouchBar(
      const std::vector<mate::PersistentDictionary>& items) override;
  void RefreshTouchBarItem(const std::string& item_id) override;
  void SetEscapeTouchBarItem(const mate::PersistentDictionary& item) override;
  gfx::Rect ContentBoundsToWindowBounds(const gfx::Rect& bounds) const override;
  gfx::Rect WindowBoundsToContentBounds(const gfx::Rect& bounds) const override;
  
  void OverrideNSWindowContentView();
  
  void SetStyleMask(bool on, NSUInteger flag);
  void SetCollectionBehavior(bool on, NSUInteger flag);
  enum class TitleBarStyle {
    NORMAL,
    HIDDEN,
    HIDDEN_INSET,
    CUSTOM_BUTTONS_ON_HOVER,
  };
  TitleBarStyle title_bar_style() const { return title_bar_style_; }