Add joinable option to SimpleThread::Options as was just done for Thread::Options.

SimpleThread will be more suitable than Thread for
https://codereview.chromium.org/2197753002/ per it not needing a MessageLoop.

Also, cleanup the SimpleThread::Options API, making it more like Thread::Options.

BUG=629716
TBR=avi@chromium.org

Review-Url: https://codereview.chromium.org/2204333003
Cr-Commit-Position: refs/heads/master@{#409830}
4 files changed