[base] Add operator<< support to StackTrace

I found this much more practical when doing some local debugging as

R=kylechar@chromium.org

StackTrace: :OutputToStream() can't be used in conjunction with DCHECK.
Change-Id: If9eb007b914a4a33d6fffb5016e0da3ada1ebc4b
Reviewed-on: https://chromium-review.googlesource.com/c/1481464
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: kylechar <kylechar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634477}

base/debug/stack_trace.cc

@@ -239,6 +239,15 @@ std::string StackTrace::ToStringWithPrefix(const char* prefix_string) const {
   return stream.str();
 }
 
+std::ostream& operator<<(std::ostream& os, const StackTrace& s) {
+#if !defined(__UCLIBC__) & !defined(_AIX)
+  s.OutputToStream(&os);
+#else
+  os << "StackTrace::OutputToStream not implemented.";
+#endif
+  return os;
+}
+
 #if BUILDFLAG(CAN_UNWIND_WITH_FRAME_POINTERS)
 
 size_t TraceStackFramePointers(const void** out_trace,

base/debug/stack_trace.h

@@ -124,6 +124,9 @@ class BASE_EXPORT StackTrace {
   size_t count_;
 };
 
+// Forwards to StackTrace::OutputToStream().
+BASE_EXPORT std::ostream& operator<<(std::ostream& os, const StackTrace& s);
+
 // Record a stack trace with up to |count| frames into |trace|. Returns the
 // number of frames read.
 BASE_EXPORT size_t CollectStackTrace(void** trace, size_t count);