Fix warnings about unreferenced local variables.

BUG=skia:2272

Change-Id: Ibf03efedc662fea2a389ad2dc5af5b7b014181a8
Reviewed-on: https://chromium-review.googlesource.com/201900
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>
diff --git a/src/common/debug.h b/src/common/debug.h
index c12fb85..8c07971 100644
--- a/src/common/debug.h
+++ b/src/common/debug.h
@@ -77,8 +77,10 @@
         ERR("\t! Assert failed in %s(%d): "#expression"\n", __FUNCTION__, __LINE__); \
         assert(expression); \
     } while(0)
+#define UNUSED_ASSERTION_VARIABLE(variable)
 #else
 #define ASSERT(expression) (void(0))
+#define UNUSED_ASSERTION_VARIABLE(variable) ((void)variable)
 #endif
 
 // A macro to indicate unimplemented functionality