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This patch makes several trivial changes:
 - Enforce GUARDED_BY against |clients_| and modify test only
   methods for that.
 - Initialize members in the header rather than constructor
   as we can as possible.
 - |finalized_| and |initialization_state_| do not have to be
   protected by |lock_|.
 - Enable GUARDED_BY in MidiManagerWinrt.

Bug: 672793
Change-Id: I726882c502b1d5b2414c5baa33ca31c3f630072e
Reviewed-on: https://chromium-review.googlesource.com/1249061Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Commit-Queue: Takashi Toyoshima <toyoshim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595398}