Use a constexpr constructor that accepts traits in any order instead of
the builder pattern to construct TaskTraits in chrome.

BUG=713683
R=gab@chromium.org
TBR=sky@chromium.org, stevenjb@chromium.org
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.win:win10_chromium_x64_rel_ng

Review-Url: https://codereview.chromium.org/2858073002
Cr-Commit-Position: refs/heads/master@{#469129}