It's about time we got rid of this. As a first step, introduce a flag,
so that some consumers may stage this change in appropriately.

BUG=chromium:534766,chromium:532048

Change-Id: Id53f0bacf5bdbf85dd71d1262d9f3a9ce3c4111f
Reviewed-on: https://boringssl-review.googlesource.com/16104
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>