Add UMA histogram for Assistant vocie match enrollment

Bug: None
Test: Local build
Change-Id: I619f49394f25c072bfa714ac0648800b89f2c729
Reviewed-on: https://chromium-review.googlesource.com/c/1484586
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Yue Li <updowndota@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635683}
4 files changed