Tidy DEFINE_(THREAD_SAFE_)STATIC_LOCAL() implementations.

Move the handling of static local singletons into the
WTF::StaticSingleton<T> wrapper class, including the "same thread"
debug verification.

Use it to also implement the "thread safe" variant also, which can
now be done in a straightforward manner after issue 686866 enabled
C++ thread safe statics.

R=kinuko
BUG=686866

Review-Url: https://codereview.chromium.org/2680843006
Cr-Commit-Position: refs/heads/master@{#451305}
4 files changed