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}

• ui/aura/BUILD.gn[diff]
• ui/aura_extra/BUILD.gn[diff]
• ui/aura_extra/window_occlusion_impl_unittest_win.cc[Added - diff]
• ui/aura_extra/window_occlusion_impl_win.cc[Added - diff]
• ui/aura_extra/window_occlusion_impl_win.h[Added - diff]
• ui/aura_extra/window_occlusion_win.cc[Added - diff]
• ui/aura_extra/window_occlusion_win.h[Added - diff]