Fix deadlock in process exit handling

runtime/bin/process.h

Index: runtime/bin/process.h
diff --git a/runtime/bin/process.h b/runtime/bin/process.h
index 04a513c7ee1a05c8c85ac8e2c841d6e7e1ca8708..f12d62493280281304de1ff9362bbb02b34d623b 100644
--- a/runtime/bin/process.h
+++ b/runtime/bin/process.h
@@ -11,6 +11,8 @@
 
 class Process {
  public:
+  // Start a new process providing access to stdin, stdout, stderr and
+  // process exit streams.
   static int Start(const char* path,
                    char* arguments[],
                    intptr_t arguments_length,
@@ -22,8 +24,12 @@ class Process {
                    char* os_error_message,
                    int os_error_message_len);
 
+  // Kill a process with a given pid.
   static bool Kill(intptr_t id);
 
+  // Indicate that the process with the given pid has exited.
+  static void Exit(intptr_t id);
+
   DISALLOW_ALLOCATION();
   DISALLOW_IMPLICIT_CONSTRUCTORS(Process);
 };