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Currently, fixing exo bugs for Lacros is cumbersome due to version
skew between Lacros/ash. In a single CL, we ideally want to 1) land
an exo fix and 2) land a conditional workaround in Lacros until ash
is uprevved to actually include the exo fix (see the bug for a real
world example).

To remedy this, this CL introduces a new |bug_fix| event that sends
down IDs of bug fixes present in exo when the zaura_shell is bound.
Clients (notably |ui::WaylandZAuraShell|) can listen for this event
and gate features on the presence of certain bug fixes by calling
|WaylandZAuraShell::HasBugFix(bug_id)|.

Bug: 1151508
Change-Id: Iec39e32c4b507545aaf59c0a73f3fa61f9a7e3ad
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2568204
Commit-Queue: Brian Ho <hob@chromium.org>
Reviewed-by: Robert Kroeger <rjkroege@chromium.org>
Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Cr-Commit-Position: refs/heads/master@{#837086}