Add user_input_monitor_unittest.cc back to BUILD.

Looks like it was accidentally removed in https://crrev.com/2577573002.

Bug: 673494

Change-Id: Ibb73e8ff0c8a1b3a222446b002eb5b7560e9fe31
Reviewed-on: https://chromium-review.googlesource.com/1012079Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Marina Ciocea <marinaciocea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551155}

media/base/BUILD.gn

@@ -485,6 +485,7 @@ source_set("unit_tests") {
     "text_ranges_unittest.cc",
     "text_renderer_unittest.cc",
     "time_delta_interpolator_unittest.cc",
+    "user_input_monitor_unittest.cc",
     "vector_math_unittest.cc",
     "video_codecs_unittest.cc",
     "video_color_space_unittest.cc",

media/base/user_input_monitor_unittest.cc

@@ -36,10 +36,6 @@ TEST(UserInputMonitorTest, CreatePlatformSpecific) {
   if (!monitor)
     return;
 
-  MockMouseListener listener;
-  // Ignore any callbacks.
-  EXPECT_CALL(listener, OnMouseMoved(testing::_)).Times(testing::AnyNumber());
-
   monitor->EnableKeyPressMonitoring();
   monitor->DisableKeyPressMonitoring();