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This eliminates MockOutShowHide and simplifies lifetime.

MockOutShowHide derived from another Mock class, and also embedded the
view inside of it. This means that the view was destroyed while the
model was being destroyed.

EXPECT_* methods are now moved to test code instead of the mock class,
since the EXPECT_* methods assume a specific type of usage that is not
always present.

ScreenManager is also cleaned up so that it does not declare any friend
tests.

Bug: 928555
Change-Id: I2f61648dc980e4d5b08620d76d3bbd4d58cd2403
Reviewed-on: https://chromium-review.googlesource.com/c/1450281Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Reviewed-by: Denis Kuznetsov <antrim@chromium.org>
Commit-Queue: Jacob Dufault <jdufault@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629264}