[Cleanup] Improve documentation for enumerated histogram best practices.

In particular:
  * Recommend using "COUNT" rather than "MAX" as the name for the enum
    boundary value, a.k.a. the enum size.
  * Use "enum class" in the docs, to encourage use in new code.
  * Include a sample blurb for documenting that the semantics of enums
    used to back UMA histograms should be time-invariant.

R=mpearson@chromium.org, rkaplow@chromium.org

Change-Id: I0466f9899bde7790dc90e14db0060968d6b5eded
Reviewed-on: https://chromium-review.googlesource.com/656431
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#506193}

CrOS-Libchrome-Original-Commit: 8811a271f4263df2db42f8efd82bc563bc8983a4
3 files changed
tree: ba480e5097af2bc95b228d9249a3d5158cc6a65a
  1. base/
  2. build/
  3. components/
  4. dbus/
  5. device/
  6. ipc/
  7. mojo/
  8. testing/
  9. third_party/
  10. ui/