Disable the TLS version fallback.

This sets the default minimum TLS fallback version to TLS 1.2. The code is
retained for now to support a resurrected SSLVersionFallbackMin admin policy.
The policy is set to expire in Chrome 53, matching the timeline for the
previous fallback removal. As an escape hatch (but I don't expect to need it),
it's also connected to a field trial.

This also tweaks the fallback code. The TLS 1.0 fallback leg is now completely
gone (the admin policy expired) and ERR_SSL_FALLBACK_BEYOND_MINIMUM_VERSION
hits have leveled off (see Net.ErrorCodesForMainFrame3), cap the fallback code
to TLS 1.1. We will no longer even try TLS 1.0 ClientHellos for the purposes of
showing the error code. This will decrease the amount of time it takes to show
an error page in some cases.

The ssl_version_fallback_min toggle is also tweaked to reject all values below
TLS 1.1, so that the resurrected admin policy cannot be used to set the value
at TLS 1.0 again. (Though it would be moot due to the above change.)

We'll also want to add a link to some to-be-written Help Center article on the
error page, but that'll be done separately after chatting with UI folks.

BUG=536200,583787

Review URL: https://codereview.chromium.org/1682623002

Cr-Commit-Position: refs/heads/master@{#377352}
9 files changed