Add VAR_TX_NO_TX_MODE experiment.

This experiment removes the tx_mode flag from the uncompressed frame
 header and instead always uses a constant value.

objective-1-fast:

master@2017-06-29T00:51:22.142Z -> no_tx_mode@2017-06-29T01:09:32.392Z
   PSNR | PSNR Cb | PSNR Cr | PSNR HVS |    SSIM | MS SSIM | CIEDE 2000
-0.0103 |  0.0183 | -0.0076 |  -0.0103 | -0.0052 | -0.0208 |     0.0004

Change-Id: I9d49c4d99ed102e52c86e339f20623063886f49d
diff --git a/av1/encoder/bitstream.c b/av1/encoder/bitstream.c
index bf60ae9..c4bd5d2 100644
--- a/av1/encoder/bitstream.c
+++ b/av1/encoder/bitstream.c
@@ -3913,6 +3913,11 @@
     *mode = ONLY_4X4;
     return;
   }
+#if CONFIG_VAR_TX_NO_TX_MODE
+  (void)wb;
+  *mode = TX_MODE_SELECT;
+  return;
+#else
 #if CONFIG_TX64X64
   aom_wb_write_bit(wb, *mode == TX_MODE_SELECT);
   if (*mode != TX_MODE_SELECT) {
@@ -3923,6 +3928,7 @@
   aom_wb_write_bit(wb, *mode == TX_MODE_SELECT);
   if (*mode != TX_MODE_SELECT) aom_wb_write_literal(wb, *mode, 2);
 #endif  // CONFIG_TX64X64
+#endif  // CONFIG_VAR_TX_NO_TX_MODE
 }
 
 #if !CONFIG_EC_ADAPT