Calculation of occlusion of native windows on Windows

This calculates the occlusion status of native windows on Windows. We plan to call this on a repeating timer to extract metrics about how often and for how long native windows are occluded.

Bug: 813093
Change-Id: I32c9b477e29657ca087e5617d30859236bd10636
Reviewed-on: https://chromium-review.googlesource.com/973186
Commit-Queue: Varun Mohan <varunmohan@google.com>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Chris Hamilton <chrisha@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#552540}

diff --git a/ui/aura/BUILD.gn b/ui/aura/BUILD.gn
index 2f74c3a9..14aa332 100644
--- a/ui/aura/BUILD.gn
+++ b/ui/aura/BUILD.gn

@@ -360,6 +360,7 @@
 test("aura_unittests") {
   sources = [
     "../compositor_extra/shadow_unittest.cc",
+    "//ui/aura_extra/window_occlusion_impl_unittest_win.cc",
     "gestures/gesture_recognizer_unittest.cc",
     "hit_test_data_provider_aura_unittest.cc",
     "mus/drag_drop_controller_mus_unittest.cc",
@@ -392,6 +393,7 @@
     "//services/ui/public/cpp",
     "//skia",
     "//testing/gtest",
+    "//ui/aura_extra",
     "//ui/base:test_support",
     "//ui/compositor:test_support",
     "//ui/compositor_extra",