Mojo: Move the header bits of MessageInTransit into a separate struct.

mojo/system/message_in_transit.cc

Index: mojo/system/message_in_transit.cc
diff --git a/mojo/system/message_in_transit.cc b/mojo/system/message_in_transit.cc
index 42b85ffc91c62ee9fe327573fd176e8edd36b104..89f1dad0777e2483db396551d183ae2ee6552d57 100644
--- a/mojo/system/message_in_transit.cc
+++ b/mojo/system/message_in_transit.cc
@@ -76,16 +76,9 @@ MessageInTransit::MessageInTransit(uint32_t data_size,
                                    Subtype subtype,
                                    uint32_t num_bytes,
                                    uint32_t num_handles)
-    : data_size_(data_size),
-      type_(type),
-      subtype_(subtype),
-      source_id_(kInvalidEndpointId),
-      destination_id_(kInvalidEndpointId),
-      num_bytes_(num_bytes),
-      num_handles_(num_handles),
-      reserved0_(0),
-      reserved1_(0) {
-  DCHECK_GE(data_size_, num_bytes_);
+    : header_(data_size, type, subtype, kInvalidEndpointId, kInvalidEndpointId,
+              num_bytes, num_handles) {
+  DCHECK_GE(header()->data_size, header()->num_bytes);
 }
 
 }  // namespace system