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}

• content/shell/browser/shell_aura.cc[diff]
• content/shell/browser/shell_platform_data_aura.cc[diff]
• content/shell/browser/shell_platform_data_aura.h[diff]
• ui/aura/BUILD.gn[diff]
• ui/aura/screen_ozone.cc[Added - diff]
• ui/aura/screen_ozone.h[Added - diff]
• ui/base/layout.cc[diff]
• ui/ozone/BUILD.gn[diff]
• ui/ozone/public/ozone_platform.cc[diff]
• ui/ozone/public/ozone_platform.h[diff]
• ui/ozone/public/platform_screen.h[Added - diff]
• webrunner/BUILD.gn[diff]
• webrunner/browser/DEPS[diff]
• webrunner/browser/webrunner_browser_main_parts.cc[diff]
• webrunner/browser/webrunner_browser_main_parts.h[diff]