C++14 requires to definition of static constexpr members to
emit a linker symbol.

Bug: chromium:819294
Change-Id: I06b7f1f7a6260982cf07a71c67295789c1f03084
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2720261Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>