Consistently order ECDHE_ECDSA over ECDHE_RSA.

Currently we don't express an opinion. Most sites aren't likely to have a
choice since it depends on what certificates they have available. But we may as
well order them.

Change-Id: I4fffa5e392f42e19823cb8faa2e9e15a6bb91086
Reviewed-on: https://boringssl-review.googlesource.com/2607
Reviewed-by: Adam Langley <agl@google.com>