Add optional workingDirectory argument to Process.start.

runtime/bin/process_linux.cc

Index: runtime/bin/process_linux.cc
===================================================================
--- runtime/bin/process_linux.cc	(revision 2981)
+++ runtime/bin/process_linux.cc	(working copy)
@@ -133,6 +133,7 @@
 int Process::Start(const char* path,
                    char* arguments[],
                    intptr_t arguments_length,
+                   const char* working_directory,
                    intptr_t* in,
                    intptr_t* out,
                    intptr_t* err,
@@ -260,6 +261,10 @@
     }
     close(read_err[1]);
 
+    if (working_directory != NULL && chdir(working_directory) == -1) {
+      ReportChildError(exec_control[1]);
+    }
+
     execvp(path, const_cast<char* const*>(program_arguments));
     ReportChildError(exec_control[1]);
   }