md_user_manager cleanup: Migrate from var to let/const.

Migration caught a case where AvatarIcon typedef was declared twice
with slightly different types.

Bug: 792774
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: Ib6ba85956dedb9d8f28c779a1e77357a643b02a9
Reviewed-on: https://chromium-review.googlesource.com/855505
Reviewed-by: Dave Schuyler <dschuyler@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#528838}
17 files changed