Add Mojo API for Blink hit testing

services/viz/public/interfaces/hit_test/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.
 
 import("//mojo/public/tools/bindings/mojom.gni")
 
 mojom("hit_test") {
   sources = [
     "hit_test_region_list.mojom",
+    "input_hit_test.mojom",
   ]
 
   public_deps = [
     "//cc/ipc:interfaces",
     "//ui/gfx/geometry/mojo",
     "//ui/gfx/mojo",
   ]
 }