Add the ability to get the current directory

runtime/bin/directory_posix.cc

Index: runtime/bin/directory_posix.cc
diff --git a/runtime/bin/directory_posix.cc b/runtime/bin/directory_posix.cc
index 7e10e52c4a88c2126bbfbab63555ca7d19b5e755..527509be343eed25535bda624f464ce64d6b7859 100644
--- a/runtime/bin/directory_posix.cc
+++ b/runtime/bin/directory_posix.cc
@@ -377,6 +377,11 @@ Directory::ExistsResult Directory::Exists(const char* dir_name) {
 }
 
 
+char* Directory::Current() {
+  return getcwd(NULL, 0);
+}
+
+
 bool Directory::Create(const char* dir_name) {
   // Create the directory with the permissions specified by the
   // process umask.