Added missing MainThreadScrollingReason in histograms.xml

ui/events/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")
 import("//ui/ozone/ozone.gni")
 
 if (is_android) {
   import("//build/config/android/rules.gni")
@@ skipping 353 matching lines @@
   ]
 
   deps = [
     ":dom_keycode_converter",
     ":events",
     ":events_base",
     ":gesture_detection",
     ":test_support",
     "//base",
     "//base/test:run_all_unittests",
+    "//base/test:test_support",
     "//ipc:test_support",
     "//skia",
     "//testing/gmock",
     "//testing/gtest",
     "//ui/events/devices",
     "//ui/events/ipc:events_ipc",
     "//ui/events/platform",
     "//ui/gfx:test_support",
   ]
 
@@ skipping 67 matching lines @@
     sources += [ "android/motion_event_android_unittest.cc" ]
   }
 }
 
 if (is_android) {
   generate_jar_jni("motionevent_jni_headers") {
     jni_package = "ui"
     classes = [ "android/view/MotionEvent.class" ]
   }
 }