define_gf_group_pass0: explicit arf_boost_factor.

Earlier arf_boost_factor was 0 implicitly and not clear from the code.
Now, made this explicit and also added a comment to explain what that
will do.

Change-Id: Ib29e6eb739e441464b9a9772d08d870aaf995b68
diff --git a/av1/encoder/pass2_strategy.c b/av1/encoder/pass2_strategy.c
index 30c653a..206deea 100644
--- a/av1/encoder/pass2_strategy.c
+++ b/av1/encoder/pass2_strategy.c
@@ -1449,6 +1449,9 @@
     rc->baseline_gf_interval = rc->frames_to_key;
 
   rc->gfu_boost = DEFAULT_GF_BOOST;
+  // This will effectively use qindex returned by 'get_gf_high_motion_quality()'
+  // for level 1 frames.
+  rc->arf_boost_factor = 0.0f;
   rc->constrained_gf_group =
       (rc->baseline_gf_interval >= rc->frames_to_key) ? 1 : 0;