[Device Service] Move Gamepad Blink headers to be part of the Gamepad client library

device/gamepad/public/cpp/BUILD.gn

+# Copyright 2017 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.
+
+# This target contains only those files that are shared by the Device Gamepad
+# implementation and all Device Gamepad clients, including Blink. Add a file
+# here only if it meets the following constraints:
+# (1) It is *necessary* to use the file to consume Device Gamepad (the
+# current files define structs that are used as a consistent interpretation of
+# shared memory by the Device Gamepad and its clients).
+# (2) The file has no dependencies on the STL or Chromium code and will not
+# grow them over time (the current files are POD structs and will necessarily
+# stay that way due to their above-described purpose).
+# Consult {platform-architecture-dev, services-dev}@chromium.org in the case
+# of any uncertainty.
+source_set("shared_with_blink") {
+  sources = [
+    "gamepad.cc",
+    "gamepad.h",
+    "gamepads.h",
+  ]
+  # Do not add deps here per the above comment.
+}