Move the |die| member of FrameThreadContext to PerThreadContext.

This fixes a data race warning by ThreadSanitizer.
FrameThreadContext.die is read by all the worker threads but is not
protected by any mutex. Move it to PerThreadContext so that each worker
thread reads its own copy of |die|, which can then be protected with
PerThreadContext.mutex.

Signed-off-by: Wan-Teh Chang <wtc@google.com>
Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>
