Use 8 threads by default for VP9 encodes

BUG=https://code.google.com/p/webm/issues/detail?id=964

Change-Id: I70679d0f139b8e0962f7e19eb56b34060953e9b0

vp9/vp9_cx_iface.c

@@ -1414,7 +1414,7 @@ static vpx_codec_enc_cfg_map_t encoder_usage_cfg_map[] = {
     0,
     {  // NOLINT
       0,                  // g_usage
-      0,                  // g_threads
+      8,                  // g_threads
       0,                  // g_profile
 
       320,                // g_width