Fix ubsan warning: test/datarate_test.cc

BUG=webm:1219

Change-Id: I48470a885cd64a60636a982cd68165c41a702306

test/datarate_test.cc

@@ -90,7 +90,7 @@ class DatarateTestLarge : public ::libvpx_test::EncoderTest,
           << pkt->data.frame.pts;
     }
 
-    const size_t frame_size_in_bits = pkt->data.frame.sz * 8;
+    const int64_t frame_size_in_bits = pkt->data.frame.sz * 8;
 
     // Subtract from the buffer the bits associated with a played back frame.
     bits_in_buffer_model_ -= frame_size_in_bits;