exo: Add support for alpha_compositing interface to wayland bindings.

ui/compositor/layer.h

Index: ui/compositor/layer.h
diff --git a/ui/compositor/layer.h b/ui/compositor/layer.h
index 26560d1d5fbd1a1f287e921c9c1323649cb738c7..8b90283a594a69c93a4238c6f0508e6beac099d2 100644
--- a/ui/compositor/layer.h
+++ b/ui/compositor/layer.h
@@ -281,6 +281,15 @@ class COMPOSITOR_EXPORT Layer
   void SetTextureFlipped(bool flipped);
   bool TextureFlipped() const;
 
+  // The alpha value applied to the whole texture. The effective value of each
+  // pixel is computed as:
+  // pixel.a = pixel.a * alpha.
+  // Note: This is different from SetOpacity() as it only applies to the
+  // texture and child layers are unaffected.
+  // TODO(reveman): Remove once components/exo code is using SetShowSurface.
+  // crbug.com/610086
+  void SetTextureAlpha(float alpha);
+
   // Begins showing content from a surface with a particular id.
   void SetShowSurface(cc::SurfaceId surface_id,
                       const cc::SurfaceLayer::SatisfyCallback& satisfy_callback,