Add a macro for unused trace variables.

This fixes the standalone 64-bit Debug build.

Change-Id: I348b71a96ada435867367e565bc84b570d2bfb67
Reviewed-on: https://chromium-review.googlesource.com/204292
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>
diff --git a/src/common/debug.h b/src/common/debug.h
index 8c07971..b6b53e4 100644
--- a/src/common/debug.h
+++ b/src/common/debug.h
@@ -83,6 +83,12 @@
 #define UNUSED_ASSERTION_VARIABLE(variable) ((void)variable)
 #endif
 
+#ifndef ANGLE_ENABLE_TRACE
+#define UNUSED_TRACE_VARIABLE(variable) ((void)variable)
+#else
+#define UNUSED_TRACE_VARIABLE(variable)
+#endif
+
 // A macro to indicate unimplemented functionality
 
 // Define NOASSERT_UNIMPLEMENTED to non zero to skip the assert fail in the unimplemented checks