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Currently, WaylandKeyboardDelegate directly subscribes
ash::ImeControllerImpl. For better layering, this CL moves
it to Keyboard, so that WaylandKeyboardDelegate will be
just a Delegate for the Keyboard.

Along with the updating mock expectation in the unittest,
this CL also updates some use of Mocks not to have a warning
and verify the behavior precisely.

BUG=1123705
TEST=Ran exo_unittests.

Change-Id: I4d889807bd8cbef3cec2e40dd0b3546a123895e0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2417672Reviewed-by: Jun Mukai <mukai@chromium.org>
Commit-Queue: Hidehiko Abe <hidehiko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#808683}