Disable SHA-1 support for Local Anchors by default

net::SSLConfig exposes a policy for configuring whether
or not to accept SHA-1 certificates issued by
locally-trusted (as opposed to publicly trusted) trust
anchors. The default for this policy was to accept these
certificates, while anything creating an
SSLConfigServiceManager under //components/ssl_config
would have these disabled by default, unless overridden
by preferences.

Change the default to be secure-by-default, as embedders
can supply an SSLConfigService that best reflects their
desired behaviours (if they do not wish the defaults) as
part of the URLRequestContext(Builder,Getter).

BUG=831240

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_mojo
Change-Id: I0bcb3474458ca4e0f3e0a554054eec3046a103bb
Reviewed-on: https://chromium-review.googlesource.com/1005416
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Richard Coles <torne@chromium.org>
Commit-Queue: Ryan Sleevi <rsleevi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549910}
5 files changed