cros: Don't send duplicate UserSession mojo messages to ash

ui/gfx/image/image_skia.h

Index: ui/gfx/image/image_skia.h
diff --git a/ui/gfx/image/image_skia.h b/ui/gfx/image/image_skia.h
index db7447d986c08a497371523456e372544a5d21be..1d1b00c44204205c7a30a82a5cce4ac432f04df5 100644
--- a/ui/gfx/image/image_skia.h
+++ b/ui/gfx/image/image_skia.h
@@ -95,6 +95,10 @@ class GFX_EXPORT ImageSkia {
   // |other|. Will also return true if both images are isNull().
   bool BackedBySameObjectAs(const gfx::ImageSkia& other) const;
 
+  // Alias for BackedBySameObjectAs(). Used by mojo EqualsTraits<> for
+  // comparing two ui::mojom::ImageSkia.
+  bool Equals(const gfx::ImageSkia& other) const;

[James Cook, 2017/04/25 00:58:28]

I think this is the right way to handle comparisons for mojo. 

Mojo has EqualsTraits<>, but they are not like StructTraits<>. I don't think
you're supposed to define your own EqualsTraits.

I could define my own mojo-only comparison like this:

namespace mojo {
namespace internals {  // uh oh, using internals

template <>
bool Equals(const ImageSkia& a, const ImageSkia& b) { ... }

}
}

However, then it's weird that ui::mojom::ImageSkia has an Equals() but the
underlying gfx::ImageSkia doesn't. That could also lead to a situation where
someone else adds gfx::ImageSkia::operator==() that doesn't match
ui::mojom::ImageSkia's concept of equals.

[xiyuan, 2017/04/25 16:04:53]

If gfx::ImageSkia needs an operation== in the future, it should probably be
BackedBySameObjectAs() as well. So I think this is fine.

[oshima, 2017/04/25 17:28:04]

Does this have to be named "Equals"? "equals" and "same" has subtle difference
in meaning so I want to use Same if possible.

[James Cook, 2017/04/25 17:32:47]

It must either be called Equals() or be operator==(). See
https://cs.chromium.org/chromium/src/mojo/public/cpp/bindings/lib/equals_trai...

I need this in order to compare two mojo structs, UserSessionPtrs.

Other options:

* Write my own comparison for the two UserSession objects, which means I have to
manually keep track of all the other fields to compare so I can do a special
comparison for the image field
* Do the hacky thing with mojo::internals mentioned above.

[James Cook, 2017/04/25 17:50:04]

Oshima, please wait on looking at this. Yuzhu had a different suggestion on how
to handles Equals(). I need to move some code in mojo.

+
   // Adds |image_rep| to the image reps contained by this object.
   void AddRepresentation(const gfx::ImageSkiaRep& image_rep);