Add basic stacktrace handler using libunwind.

gm/gmmain.cpp

Index: gm/gmmain.cpp
diff --git a/gm/gmmain.cpp b/gm/gmmain.cpp
index 37237695c04a60f02c79a96d88bfd11823df5ee5..f4cd2c1da7ce29aac64bbfddc00d89fe80d3af47 100644
--- a/gm/gmmain.cpp
+++ b/gm/gmmain.cpp
@@ -17,6 +17,7 @@
 #include "gm_error.h"
 #include "gm_expectations.h"
 #include "system_preferences.h"
+#include "CrashHandler.h"
 #include "SkBitmap.h"
 #include "SkColorPriv.h"
 #include "SkCommandLineFlags.h"
@@ -2229,6 +2230,7 @@ static bool parse_flags_jpeg_quality() {
 
 int tool_main(int argc, char** argv);
 int tool_main(int argc, char** argv) {
+    SetupCrashHandler();
 
     SkString usage;
     usage.printf("Run the golden master tests.\n");