libvpx: Pull from upstream

source/libvpx/vpx/vpx_frame_buffer.h

Index: source/libvpx/vpx/vpx_frame_buffer.h
diff --git a/source/libvpx/vpx/vpx_frame_buffer.h b/source/libvpx/vpx/vpx_frame_buffer.h
index 41038b10df6726fae01341cb14e5c44f44e7697a..9036459af0a344690780712e15b97a3c66161d29 100644
--- a/source/libvpx/vpx/vpx_frame_buffer.h
+++ b/source/libvpx/vpx/vpx_frame_buffer.h
@@ -22,8 +22,11 @@ extern "C" {
 #include "./vpx_integer.h"
 
 /*!\brief The maximum number of work buffers used by libvpx.
+ *  Support maximum 4 threads to decode video in parallel.
+ *  Each thread will use one work buffer.
+ * TODO(hkuang): Add support to set number of worker threads dynamically.
  */
-#define VPX_MAXIMUM_WORK_BUFFERS 1
+#define VPX_MAXIMUM_WORK_BUFFERS 8
 
 /*!\brief The maximum number of reference buffers that a VP9 encoder may use.
  */