Add basic stacktrace handler using libunwind.

This means we will all have to apt-get install libunwind8-dev on Linux.  Mac comes with everything we need already.

BUG=skia:
R=reed@google.com, mtklein@google.com

Author: mtklein@chromium.org

Review URL: https://codereview.chromium.org/343583005
diff --git a/dm/DM.cpp b/dm/DM.cpp
index 776b642..0de6616 100644
--- a/dm/DM.cpp
+++ b/dm/DM.cpp
@@ -9,6 +9,7 @@
 #include "SkString.h"
 #include "Test.h"
 #include "gm.h"
+#include "CrashHandler.h"
 
 #include "DMBenchTask.h"
 #include "DMCpuGMTask.h"
@@ -215,6 +216,7 @@
 
 int tool_main(int argc, char** argv);
 int tool_main(int argc, char** argv) {
+    SetupCrashHandler();
     SkAutoGraphics ag;
     SkCommandLineFlags::Parse(argc, argv);