vp9_pickmode: Fix signed/unsigned mismatch.

Avoids getting reverted on the next libvpx chrome roll attempt.

Change-Id: I2431bd601111a78320af5f211f6413009674f1c8

vp9/encoder/vp9_pickmode.c

diff --git a/vp9/encoder/vp9_pickmode.c b/vp9/encoder/vp9_pickmode.c
index ad77cd0..adaa044 100644
--- a/vp9/encoder/vp9_pickmode.c
+++ b/vp9/encoder/vp9_pickmode.c
@@ -161,8 +161,8 @@
   struct macroblock_plane *const p = &x->plane[0];
   struct macroblockd_plane *const pd = &xd->plane[0];
 
-  int var = cpi->fn_ptr[bsize].vf(p->src.buf, p->src.stride,
-                                  pd->dst.buf, pd->dst.stride, &sse);
+  unsigned int var = cpi->fn_ptr[bsize].vf(p->src.buf, p->src.stride,
+                                           pd->dst.buf, pd->dst.stride, &sse);
 
   // TODO(jingning) This is a temporary solution to account for frames with
   // light changes. Need to customize the rate-distortion modeling for non-RD