[Autofill Assistant] Added missing labels to AutofillAssistantDropOutReason for UMA.

Note: the client side enum is here: http://cs/clankium/src/components/autofill_assistant/browser/metrics.h

Bug: 806868
Change-Id: I08daeb80035ace94c9934e31f779d165d15eb92b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1533908
Reviewed-by: Mathias Carlen <mcarlen@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Commit-Queue: Clemens Arbesser <arbesser@google.com>
Cr-Commit-Position: refs/heads/master@{#643368}
1 file changed