Avoid checking for WTFThreadData::staticData in wtfThreadData()

This patch initialized the underlying ThreadSpecific<WTFThreadData>*
in a separate code path during initialization, to avoid a branch in
wtfThreadData().

BUG=621786

Review-Url: https://codereview.chromium.org/2646003003
Cr-Commit-Position: refs/heads/master@{#447897}
4 files changed