Add a mojom/StructTrait for display::Display.

ui/display/mojo/display.mojom

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+module display.mojom;
+
+import "ui/gfx/geometry/mojo/geometry.mojom";
+
+// Corresponds to display::Display::Rotation.

[sky, 2016/07/06 16:40:34]

The reason I added FrameDecorationValues to Display is that nearly all mus
clients need to know the FrameDecorationValues so that they can create windows.
That said, I don't think we should create duplicate structures. Instead I
recommend moving FrameDecorationValues out of Display and have the function that
supplies FrameDecorationValues to provide it as an additional argument (or use a
struct with FrameDecorationValues and Display). That way we can have one display
class that is used by both.

[kylechar, 2016/07/06 17:19:19]

It's not really intended to keep duplicate structures, this is just a first to
add the 1:1 corresponding mojom. I don't think it's really possible to have
extra fields in a mojom and use a StructTraits, because those extra fields won't
be serialized/deserialized?

From here I could make a C++ MusDisplay (or something like that) class:

public MusDisplay : display::Display {
 public:
   // Details.
 private:
  FrameDecorationValues values;
}

And make a mojom struct:

struct MusDisplay {
  display.mojom.Display display;
  FrameDecorationvalues values;
}

The StructTraits for the mojom<->class is trivial at that point. Mus can then
use the new C++ class directly instead of the ui::mojom::DisplayPtr which should
make life a bit nicer.

+enum DisplayRotation {
+  VALUE_0,
+  VALUE_90,
+  VALUE_180,
+  VALUE_270,
+};
+
+// Corresponds to display::Display::TouchSupport.
+enum TouchSupport {
+  UNKNOWN,
+  AVAILABLE,
+  UNAVAILABLE,
+};
+
+// Corresponds display ui::Display.
+struct Display {
+  int64 id;
+  gfx.mojom.Rect bounds;
+  gfx.mojom.Rect work_area;
+  float device_scale_factor;
+  DisplayRotation rotation;
+  TouchSupport touch_support;
+  gfx.mojom.Size maximum_cursor_size;
+};