[Cleanup] Define TaskTraits as constexpr.

BUG=667892
R=gab@chromium.org

Change-Id: If4c7bdd1a8109a38cf0dc1081234e025e267f217
Reviewed-on: https://chromium-review.googlesource.com/570732
Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#486935}
1 file changed