update vp9_thread.[hc]

update vp9_thread.[hc]

pull the latest from WebP, which adds a worker interface abstraction
allowing an application to override init/reset/sync/launch/execute/end

this has the side effect of removing a harmless, but annoying, TSan
warning.

Original source:
 http://git.chromium.org/webm/libwebp.git
  100644 blob 08ad4e1fecba302bf1247645e84a7d2779956bc3 src/utils/thread.c
  100644 blob 7bd451b124ae3b81596abfbcc823e3cb129d3a38 src/utils/thread.h

Local modifications:
 - s/WebP/VP9/g
 - camelcase functions -> lower with _'s
 - associate '*' with the variable, not the type

Change-Id: I875ac5a74ed873cbcb19a3a100b5e0ca6fcd9aed