Previously, we tested that glibc's cached PID was correct by relying on
an assert inside of pthread_getattr_np. This relies on a fairly
implementation-specific detail of pthread_getattr_np and is also
problematic, as glibc's pthread_getattr_np relies on reading
/proc/self/maps when called from the main thread (but the test chroots).
This change inspects the cached PID more directly using getpid instead.
BUG=646191
Review-Url: https://codereview.chromium.org/2474703003
Cr-Commit-Position: refs/heads/master@{#429369}
0473e4c06b