Rollback trunk to 3.21.15.

src/platform-macos.cc

Index: src/platform-macos.cc
diff --git a/src/platform-macos.cc b/src/platform-macos.cc
index a58bc1aca3943184d0185246c5498a060b90149b..67cc96f93790a93692bce5f9ad0e51d66a328af9 100644
--- a/src/platform-macos.cc
+++ b/src/platform-macos.cc
@@ -220,6 +220,14 @@ double OS::LocalTimeOffset() {
 }
 
 
+int OS::StackWalk(Vector<StackFrame> frames) {
+  // If weak link to execinfo lib has failed, ie because we are on 10.4, abort.
+  if (backtrace == NULL) return 0;
+
+  return POSIXBacktraceHelper<backtrace, backtrace_symbols>::StackWalk(frames);
+}
+
+
 VirtualMemory::VirtualMemory() : address_(NULL), size_(0) { }