Mojo: Modify plumbing for the WaitMany implementation.

mojo/system/core.h

Index: mojo/system/core.h
diff --git a/mojo/system/core.h b/mojo/system/core.h
index ba811f878479edfd92af4d917854b7f35b3c84d7..363b89732c3d3b15de427bb53b3314172e185d5a 100644
--- a/mojo/system/core.h
+++ b/mojo/system/core.h
@@ -108,11 +108,14 @@ class MOJO_SYSTEM_IMPL_EXPORT Core {
   friend bool internal::ShutdownCheckNoLeaks(Core*);
 
   // Internal implementation of |Wait()| and |WaitMany()|; doesn't do basic
-  // validation of arguments.
+  // validation of arguments. |*result_index| is only set if the result (whether
+  // success or failure) applies to a specific handle, so its value should be
+  // preinitialized to |static_cast<uint32_t>(-1)|.
   MojoResult WaitManyInternal(const MojoHandle* handles,
                               const MojoHandleSignals* signals,
                               uint32_t num_handles,
-                              MojoDeadline deadline);
+                              MojoDeadline deadline,
+                              uint32_t* result_index);
 
   // ---------------------------------------------------------------------------