Add observer for cert provider signature requests

Add an observer class that allows other components of Chrome to know
when a signature is successfully generated via the
chrome.certificateProvider API.

This will be used in follow-up CLs for implementing the smart card based
user login feature: this observer will be used to detect the fact of
the smart card being used during the sign-in, and to obtain the actual
certificate blob.

Bug: 826417
Test: unit_tests --gtest_filter=*CertificateProvider*
Change-Id: I745463a470d1c697682c5d76da17b52a40a24cde
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1666628
Commit-Queue: Maksim Ivanov <emaxx@chromium.org>
Reviewed-by: Pavol Marko <pmarko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672022}
5 files changed