Set palette_throughput experiment default on.

Change-Id: I6c0897f9d432121c425ae1cea34bd7f72d207c72

configure

diff --git a/configure b/configure
index 468a1b9..0d07b52 100755
--- a/configure
+++ b/configure
@@ -497,6 +497,7 @@
     soft_enable ec_multisymbol
     soft_enable palette
     soft_enable alt_intra
+    soft_enable palette_throughput
 
     # Fix up experiment dependencies
     enabled pvq && enable_feature ec_adapt