Implement gfx::SelectionBound StructTraits

ui/gfx/mojo/BUILD.gn

 # 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.
 
 import("//mojo/public/tools/bindings/mojom.gni")
 
 mojom("mojo") {
   sources = [
+    "selection_bound.mojom",
     "transform.mojom",
   ]
+
+  public_deps = [
+    "//ui/gfx/geometry/mojo",
+  ]
 }
 
 mojom("test_interfaces") {
   sources = [
     "traits_test_service.mojom",
   ]
 
   public_deps = [
     ":mojo",
   ]
 }