Add StackTrace::ToString().

base/debug/stack_trace.cc

Index: base/debug/stack_trace.cc
diff --git a/base/debug/stack_trace.cc b/base/debug/stack_trace.cc
index 94ce145dfa18ef1ba00b330874b4c651a649c415..1919abcef1fc9930c71c62b0a2529089d98edbb6 100644
--- a/base/debug/stack_trace.cc
+++ b/base/debug/stack_trace.cc
@@ -9,6 +9,7 @@
 #include <string.h>
 
 #include <algorithm>
+#include <sstream>
 
 namespace base {
 namespace debug {
@@ -30,5 +31,11 @@ const void *const *StackTrace::Addresses(size_t* count) const {
   return NULL;
 }
 
+std::string StackTrace::ToString() const {
+  std::stringstream stream;
+  OutputToStream(&stream);
+  return stream.str();
+}
+
 }  // namespace debug
 }  // namespace base