Unified: Add UMA metric for managed device help page button.

Records metric for the number of clicks to open the help for managed
device. Currently only enterprise-managed device shows such button.
The variable is made to enum because we are likely to add same button
for Family Link managed account.

Bug: 885114
Change-Id: Ib508686737c0a17cbf0385a39f4d45b2fef863f7
Reviewed-on: https://chromium-review.googlesource.com/c/1319404
Reviewed-by: Tetsui Ohkubo <tetsui@chromium.org>
Reviewed-by: Jesse Doherty <jwd@chromium.org>
Commit-Queue: Tatsuhisa Yamaguchi <yamaguchi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606336}