Move unit tests for ClientControlledShellSurface into separate file.

BUG=773461
TEST=None

Change-Id: I2dd813cd873e7b437606085784543b1a3d72dc95
Reviewed-on: https://chromium-review.googlesource.com/801090Reviewed-by: David Reveman <reveman@chromium.org>
Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521523}

components/exo/BUILD.gn

@@ -127,6 +127,7 @@ source_set("unit_tests") {
   sources = [
     "../../ui/events/ozone/gamepad/gamepad_event.cc",
     "buffer_unittest.cc",
+    "client_controlled_shell_surface_unittest.cc",
     "data_device_unittest.cc",
     "data_offer_unittest.cc",
     "data_source_unittest.cc",