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This cl adds an initial display fetching support for Ozone/Wayland:
  - First, DesktopScreenOzone adds itself as an observer to a
    NativeDisplayDelegate, which has its own implementation based on
    a platform.
  - Then, DesktopScreenOzone initializes NativeDisplayDelegate and,
    in case of WaylandNativeDisplayDelegate, WaylandNativeDisplayDelegate
    sets itself as an observer to a PrimaryOutput.
  - Once PrimaryOutput creates current display's snapshot and current mode
    , (WaylandNativeDisplayDelegate::)OnOutputReadyForUse is called,
    WaylandNativeDisplayDelegate notifies its observers (DesktopScreenOzone
    in this case) through OnConfigurationChanged method.
  - Then, DesktopScreenOzone calls NativeDisplayDelegate::GetDisplays,
    which returns a vector of display snapshots through a callback, and
    DesktopScreenOzone::OnHostDisplaysReady creates a display::Display
    with parameters it gets from display::DisplaySnapshot.

Preliminary support for mode changes has also been added. If we take a
normal run-time browser flow, WaylandOutput::OutputHandleMode can be
called on every display change (it can be a resolution change, a refresh rate
change and etc). Once |current_mode_| is updated, OnOutputReadyForUse is
again called and DesktopScreenOzone fetches the newest list of displays,
which is then used to create a display::Display with the ids provided.
As a result, ProcessDisplayChanged is called and stored display::Display
updated by checking its id.

Note: it's impossible to run Wayland without --in-process-gpu flag
as long as support for gpu process communication with wayland
connection is not in place yet, but it's under development and
going to be pushed soon.

To try this, compile with the following gn args
use_ozone = true
ozone_platform_wayland = true

TEST=./out/Debug/chrome --ozone-platform=wayland --in-process-gpu

Bug: 578890
Change-Id: Ia0f61f1af24eeb46433d22cf31f69396325e4c78
Reviewed-on: https://chromium-review.googlesource.com/1051829Reviewed-by: Robert Kroeger <rjkroege@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Maksim Sisov <msisov@igalia.com>
Cr-Commit-Position: refs/heads/master@{#559446}