Remove unused include_dir in //ui/event/BUILD.gn

An include_dir setup for gmock in //ui/event/BUILD.gn is unused, and
that prevents to add another include dir to gmock dependencies.
This CL removes it for the preparation.

BUG=630299

Review-Url: https://codereview.chromium.org/2171793002
Cr-Commit-Position: refs/heads/master@{#406909}

ui/events/BUILD.gn

@@ -408,8 +408,6 @@ test("events_unittests") {
     "//third_party/mesa:osmesa",
   ]
 
-  include_dirs = [ "//testing/gmock/include" ]
-
   if (use_x11) {
     sources += [
       "devices/x11/device_data_manager_x11_unittest.cc",