Implemented IsFullscreen for Ozone.

This CL proposes a very simple approach to determine whether he fullscreen
mode is currently enabled: we simply check whether any browser has its
main window returning true from its IsFullscreen() method.  This is all we
can do in secure environments where an application cannot know anything
about other applications.

R=msisov@igalia.com, rjkroege@chromium.org, spang@chromium.org

Bug: 934711
Change-Id: I54f3c1bf4c8df3472d0283b2b2045cf15c173578
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1697607
Auto-Submit: Alexander Dunaev <adunaev@igalia.com>
Reviewed-by: Marc Treib <treib@chromium.org>
Reviewed-by: Maksim Sisov <msisov@igalia.com>
Reviewed-by: Robert Kroeger <rjkroege@chromium.org>
Reviewed-by: Michael Spang <spang@chromium.org>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#678615}