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WaylandConnection initialization is considered done once at least one of
the wl_outputs advertised gets its "done" event (ie: all its metadata
was already received, ie: size, origin, mode, etc), which has been
working well so far, at least with the Wayland compositors tested until
now.

It turns out that such initialization is broken for multi-screen
scenarios, where the compositor sends events in the following order:

...
wl_registry::global for wl_output object 1
wl_registry::global for wl_output object 2
wl_output::geometry for wl_output 1
wl_output::mode for wl_output 1        <-- output 1 ready
wl_output::geometry for wl_output 2
wl_output::mode for wl_output 2        <-- output 2 ready
...

This is how Exo compositor sends the events, which leads Ozone/Wayland
to crash at startup. To fix it, this patch modifies code that deals with
wl_output and WaylandScreen so that it only creates display::Display
objects for wl_outputs once they are "ready" (ie: all required metadata
already received from the server). In addition, some cleanup and minor
fixes are made.

R=tonikitoo@igalia.com

Bug: 1111905
Test: Covered by unit tests.
Change-Id: I25d23c5a76d0b2ab900bab4460344cf908db07e9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2332844
Commit-Queue: Nick Yamane <nickdiego@igalia.com>
Reviewed-by: Maksim Sisov (GMT+3) <msisov@igalia.com>
Reviewed-by: Antonio Gomes (GMT-4) <tonikitoo@igalia.com>
Cr-Commit-Position: refs/heads/master@{#794255}