Native themes: Add menu separator part

This CL:
* Adds themeable menu separators on platforms other than Windows
* Removes menu_separator_win
* Adds gtk-drawn menu separators on Linux

Besides refactoring, menu separators on Windows, CrOS, and Mac should be
unchanged.

BUG=132847
[email protected],[email protected]

Review-Url: https://codereview.chromium.org/2655553003
Cr-Commit-Position: refs/heads/master@{#446911}
diff --git a/ui/native_theme/native_theme_base.h b/ui/native_theme/native_theme_base.h
index fa06cfbc..ca838d49 100644
--- a/ui/native_theme/native_theme_base.h
+++ b/ui/native_theme/native_theme_base.h
@@ -104,6 +104,12 @@
       const gfx::Rect& rect,
       const MenuItemExtraParams& menu_item) const;
 
+  virtual void PaintMenuSeparator(
+      SkCanvas* canvas,
+      State state,
+      const gfx::Rect& rect,
+      const MenuSeparatorExtraParams& menu_separator) const;
+
   virtual void PaintSliderTrack(
       SkCanvas* canvas,
       State state,