Remove CredentialsContainer.requireUserMediation

This change removes CredentialsContainer.requireUserMediation in favor
of CredentialsContainer.preventSilentAccess. This feature was deprecated
in M60 and slanted for removal in M62.

Intent to Deprecate:
https://groups.google.com/a/chromium.org/d/msg/blink-dev/TZ1wr5BJ0Uk/1NUDJOSUCAAJ

Bug: 724464
Change-Id: I5ab2334733c4b8ab9f67af1c66de5f666d205719
Reviewed-on: https://chromium-review.googlesource.com/779195
Reviewed-by: Mike West <mkwst@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518189}
9 files changed