Add struct_traits and typemap for blink::WebGamepad

device/gamepad/public/interfaces/gamepad.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 device.mojom;
+
+struct GamepadQuaternion {
+  float x;
+  float y;
+  float z;
+  float w;
+};
+
+struct GamepadVector {
+  float x;
+  float y;
+  float z;
+};
+
+struct GamepadButton {
+  bool pressed;
+  bool touched;
+  double value;
+};
+
+struct GamepadPose {
+  GamepadQuaternion? orientation;
+  GamepadVector? position;
+  GamepadVector? angular_velocity;
+  GamepadVector? linear_velocity;
+  GamepadVector? angular_acceleration;
+  GamepadVector? linear_acceleration;
+};
+
+enum GamepadHand {
+  GamepadHandNone = 0,
+  GamepadHandLeft = 1,
+  GamepadHandRight = 2
+};
+
+struct Gamepad {
+  bool connected;
+  array<uint16, 128> id;  //blink::WebGamepad::idLengthCap = 128
+  uint64 timestamp;
+  uint32 axes_length;

[yzshen1, 2016/11/15 19:17:33]

If axes length is variable, how about making |axes| *not* fixed-size and
eliminate |axes_length|? (and also |buttons_length|) It is a waste to pass
unnecessary bytes.

We could verify that they don't exceed the max length in the struct traits.

Question: are |id| and |mapping| of variable size too? I noticed the word "Cap"
in the comment.

[ke.he, 2016/11/16 09:51:40]

Yes, to avoid passing unnecessary bytes, |axes_length| and |buttons_length|
should be eliminated.
As for |id| and |mapping|, they are "fixed array"s which are used to place
string(WebUChars), the "Cap" are their capacity. so we can pass only the
no-empty part of the fixed-array to avoid wastes.

+  array<double, 16> axes;  //blink::WebGamepad::axesLengthCap = 16
+  uint32 buttons_length;
+  array<GamepadButton, 32> buttons;  //blink::WebGamepad::buttonLengthCap = 32
+  array<uint16, 16> mapping;  //blink::WebGamepad::mappingLengthCap = 16
+  GamepadPose? pose;
+  GamepadHand hand;
+  uint32 display_id;
+};