Revise TLS 1.3 downgrade feature and metrics.

Due to non-compliant middleboxes, it is possible we'll need to do some
surgery to this mechanism. Fix the double-negative on the base::Feature
so it is less confusing and gather more extensive metrics when
enforcement is disabled. Also flip the default in trunk to disabled and
connect it to about:flags.

Finally, add a bunch of machinery to test this stuff.

Bug: boringssl:226
Change-Id: Ib910328c9733b47db16b4cb811e8f2cb09b69d0c
Reviewed-by: Ilya Sherman <>
Reviewed-by: Steven Valdez <>
Commit-Queue: David Benjamin <>
Cr-Original-Commit-Position: refs/heads/master@{#599879}
Cr-Mirrored-Commit: 570460e9b6482055b236a8b60ef5521506bf991c
4 files changed