Mac build fix

base/debug_util_posix.cc

Index: base/debug_util_posix.cc
diff --git a/base/debug_util_posix.cc b/base/debug_util_posix.cc
index 466bb0e76181367ec282bf32089bd2984f33b34b..5594737c4a40fe983d07093030f02b1bf4dad02b 100644
--- a/base/debug_util_posix.cc
+++ b/base/debug_util_posix.cc
@@ -119,7 +119,7 @@ void StackTrace::PrintBacktrace() {
 // TODO(port): complete this code
 StackTrace::StackTrace() { }
 
-StackTrace::PrintBacktrace() {
+void StackTrace::PrintBacktrace() {
   NOTIMPLEMENTED();
 }