//ui/message_center: make NotifierType a scoped enum

Also get rid of the SIZE placeholder constant, since it requires
additional case statements in switches. Define kMaxValue instead: clang
enforces the correct definition of this value, and the UMA enumeration
histograms can take advantage of it without any weird casting.

Bug: 742517
Tbr: oshima@chromium.org
Tbr: thestig@chromium.org
Change-Id: I34373733f7be363aa1d70827ae88dc03c6934819
Reviewed-on: https://chromium-review.googlesource.com/c/1312685
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605831}