Move String::isSafeToSendToAnotherThread to StringImpl.

String should be a thin wrapper over StringImpl and other functions.

I also added isSafeToSendToAnotherThread() to AtomicString since this method is
useful there too.

Review-Url: https://codereview.chromium.org/2335573003
Cr-Commit-Position: refs/heads/master@{#418115}
4 files changed