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/tests/skia_test.cpp b/tests/skia_test.cpp
index b7fbfc6..f70a7fa 100644
--- a/tests/skia_test.cpp
+++ b/tests/skia_test.cpp
@@ -5,6 +5,7 @@
  * found in the LICENSE file.
  */
 
+#include "CrashHandler.h"
 #include "OverwriteLine.h"
 #include "SkCommandLineFlags.h"
 #include "SkGraphics.h"
@@ -132,6 +133,7 @@
 
 int tool_main(int argc, char** argv);
 int tool_main(int argc, char** argv) {
+    SetupCrashHandler();
     SkCommandLineFlags::SetUsage("");
     SkCommandLineFlags::Parse(argc, argv);
     Test::SetResourcePath(FLAGS_resourcePath[0]);