Add a missing file in cc/BUILD.gn

cc/BUILD.gn

 # Copyright 2014 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("//build/config/ui.gni")
 import("//testing/test.gni")
 
 component("cc") {
   sources = [
     "animation/animation.cc",
@@ skipping 97 matching lines @@
     "debug/unittest_only_benchmark.h",
     "debug/unittest_only_benchmark_impl.cc",
     "debug/unittest_only_benchmark_impl.h",
     "input/browser_controls_offset_manager.cc",
     "input/browser_controls_offset_manager.h",
     "input/browser_controls_offset_manager_client.h",
     "input/input_handler.cc",
     "input/input_handler.h",
     "input/layer_selection_bound.cc",
     "input/layer_selection_bound.h",
+    "input/main_thread_scrolling_reason.h",
     "input/page_scale_animation.cc",
     "input/page_scale_animation.h",
     "input/scroll_elasticity_helper.cc",
     "input/scroll_elasticity_helper.h",
     "input/scroll_state.cc",
     "input/scroll_state.h",
     "input/scroll_state_data.cc",
     "input/scroll_state_data.h",
     "input/scrollbar_animation_controller.cc",
     "input/scrollbar_animation_controller.h",
@@ skipping 951 matching lines @@
     "//ui/gl:test_support",
   ]
 
   # This target should not require the Chrome executable to run.
   assert_no_deps = [ "//chrome" ]
 
   data = [
     "test/data/",
   ]
 }