update vp9_thread.c

update vp9_thread.c

pull the latest from libwebp.

Original source:
 http://git.chromium.org/webm/libwebp.git
 100644 blob 264210ba2807e4da47eb5d18c04cf869d89b9784 src/utils/thread.c

commit 46fd44c1042c9903b2f1ab87e9f200a13c7e702d
Author: James Zern <jzern@google.com>
Date:   Tue Jul 8 19:53:28 2014 -0700

    thread: remove harmless race on status_ in End()

    if a thread was still doing work when End() was called there'd be a race
    on worker->status_. in these cases, however, the specific value is
    meaningless as it would be >= OK and the thread would have been shut
    down properly, but we'll check 'impl_' instead to avoid any potential
    TSan/DRD reports.

    Change-Id: Ib93cbc226a099f07761f7bad765549dffb8054b1

Change-Id: Ib0ef25737b3c6d017fa74822e21ed58508230b91