Add PlatformScreen interface in Ozone.

Added OzonePlatform::CreateScreen(), which returns in instance of the
new PlatformScren interface. Also added aura::SceenOzone that implements
display::Screen on top of PlatformScreen.

Bug: 850650
Change-Id: I3481dda921f564f0c117d9c8a0e8ce060254f4c3
Reviewed-on: https://chromium-review.googlesource.com/1162354
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Robert Kroeger <rjkroege@chromium.org>
Reviewed-by: Michael Spang <spang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582792}

diff --git a/ui/aura/BUILD.gn b/ui/aura/BUILD.gn
index 578427b..8c5bf69 100644
--- a/ui/aura/BUILD.gn
+++ b/ui/aura/BUILD.gn

@@ -208,6 +208,8 @@
 
   if (use_ozone) {
     deps += [ "//ui/ozone" ]
+    public += [ "screen_ozone.h" ]
+    sources += [ "screen_ozone.cc" ]
   }
 
   if (is_android) {