vp9_thread: add vp9_worker_execute()

vp9_thread: add vp9_worker_execute()

cherry-picked from:
commit 988b70844e03efcfcc075a9bc25d846670494f36
Author: Pascal Massimino <pascal.massimino@gmail.com>
Date:   Fri Aug 2 11:15:16 2013 -0700

    add WebPWorkerExecute() for convenient bypass

    This is mainly for re-using the worker structs without using the
    thread.

    Change-Id: I8e1be29e53874ef425b15c192fb68036b4c0a359

Original source:
 http://git.chromium.org/webm/libwebp.git
 100644 blob c0d318aee628fdf9ba4876451a28aa978f1066b8 src/utils/thread.c
 100644 blob c2b92c9fe353f8e514f78922f3d237204a9cbc66 src/utils/thread.h

Change-Id: I13fe92b1e94062bb99fdeeb7cb0b4b0575d27793