Only send the last blur vote per form signature

Clients have a quota of one blur vote per form signature within
GetThrottleResetPeriod() days. This is not ideal because the user can
fill one field, unfocus the form, and then continue filling the fields.
In this situation, no blur votes would be sent for any of the other
fields.

After this CL, blur votes are cached, keyed by form signature. If a new
blur vote arrives for a form signature, it overrides the previous one.
For submitted forms, we don't send blur votes anymore.

Bug: 1383502
Change-Id: I498ddab432c947e682a2787f50e4d4ecb19887c6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4041382
Reviewed-by: Christoph Schwering <schwering@google.com>
Commit-Queue: Dominic Battré <battre@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1075641}