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It is just the delegate of Keyboard, so instead of using
the observer, this CL just defers the lifetime management
of WaylandKeyboardDelegate to Keyboard.
This allows us to remove the inheritance of KeyboardObserver
from WaylandKeyboardDelegate.

BUG=1123705
TEST=Ran exo_unittests.

Change-Id: I45b35a016edc3c7bd441223f1b25a28ad1debaa3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2416476
Commit-Queue: Hidehiko Abe <hidehiko@chromium.org>
Reviewed-by: Jun Mukai <mukai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#808192}