Add last OS error into heap stats.

src/platform-posix.cc

Index: src/platform-posix.cc
diff --git a/src/platform-posix.cc b/src/platform-posix.cc
index 613e8e4051f79ca80a7804340a5d023aa7f02653..c50d396ad3822542dbaf75c56986913d5c412618 100644
--- a/src/platform-posix.cc
+++ b/src/platform-posix.cc
@@ -108,6 +108,11 @@ double OS::DaylightSavingsOffset(double time) {
 }
 
 
+int OS::GetLastError() {
+  return errno;
+}
+
+
 // ----------------------------------------------------------------------------
 // POSIX stdio support.
 //