Rollback trunk to 3.21.15.

src/platform-cygwin.cc

Index: src/platform-cygwin.cc
diff --git a/src/platform-cygwin.cc b/src/platform-cygwin.cc
index 5903438932b61ad6eea29c7cac5441bca78c632a..4d3b1e313e631d8e0ba498c11bc35b257cebee47 100644
--- a/src/platform-cygwin.cc
+++ b/src/platform-cygwin.cc
@@ -205,6 +205,12 @@ void OS::SignalCodeMovingGC() {
 }
 
 
+int OS::StackWalk(Vector<OS::StackFrame> frames) {
+  // Not supported on Cygwin.
+  return 0;
+}
+
+
 // The VirtualMemory implementation is taken from platform-win32.cc.
 // The mmap-based virtual memory implementation as it is used on most posix
 // platforms does not work well because Cygwin does not support MAP_FIXED.