The type std::enable_if<cond> does always exist, it only makes sense to
check for std::enable_if<cond>::type. But the way this is used here we
also cannot do that, so just replace this by a good old "#ifdef DEBUG".

Drive-by: Minor unrelated cleanup (constexpr and ifdef).

R=eholk@chromium.org

Change-Id: I6bc27ee3adfd3ec3d38d61df67dd9cdff0faf2f7
Reviewed-on: https://chromium-review.googlesource.com/939387Reviewed-by: Eric Holk <eholk@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51612}