Delete Mac & Cairo skia::BitmapPlatformDevice impls

skia/ext/platform_device.h

Index: skia/ext/platform_device.h
diff --git a/skia/ext/platform_device.h b/skia/ext/platform_device.h
index b756aa613a30bbd2e9bfba083b3beecf0d34ee13..f349a4972137aa9a7203b80070dafa2b0e645c03 100644
--- a/skia/ext/platform_device.h
+++ b/skia/ext/platform_device.h
@@ -46,11 +46,14 @@ class SK_API PlatformDevice {
  public:
   virtual ~PlatformDevice();
 
+// Only implemented in bitmap_platform_device_win.
+#if defined(WIN32)
   // The DC that corresponds to the bitmap, used for GDI operations drawing
   // into the bitmap. This is possibly heavyweight, so it should be existant
   // only during one pass of rendering.
   virtual NativeDrawingContext BeginPlatformPaint(const SkMatrix& transform,
                                                   const SkIRect& clip_bounds) = 0;
+#endif
 };
 
 }  // namespace skia