Mojo: (Theoretically) implement the read side of platform handle passing.

mojo/system/message_in_transit.h

Index: mojo/system/message_in_transit.h
diff --git a/mojo/system/message_in_transit.h b/mojo/system/message_in_transit.h
index 8b9fbbba9b38e27b3c2f07aa00d70f84349a0409..66bb4560d23766f37d48bdf9f66f0a7624788065 100644
--- a/mojo/system/message_in_transit.h
+++ b/mojo/system/message_in_transit.h
@@ -88,7 +88,8 @@ class MOJO_SYSTEM_IMPL_EXPORT MessageInTransit {
     // It returns true (and leaves |error_message| alone) if this object appears
     // to be a valid message (according to the above) and false, pointing
     // |*error_message| to a suitable error message, if not.
-    bool IsValid(const char** error_message) const;
+    bool IsValid(size_t serialized_platform_handle_size,
+                 const char** error_message) const;
 
     // API parallel to that for |MessageInTransit| itself (mostly getters for
     // header data).