[Autofill] Limit number of elements in strike database

This CL adds the possibility to limit the number of elements in the
strike database to limit the memory footprint of the mechanism.

It also introduces constness where possible.



Change-Id: Ic231ef19016da8054e8ad65d7b70e8d9757dfb73
Bug: 1175693
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2876887
Commit-Queue: Matthias Körber <koerber@google.com>
Reviewed-by: Jared Saul <jsaul@google.com>
Reviewed-by: Christoph Schwering <schwering@google.com>
Cr-Commit-Position: refs/heads/master@{#880866}