Initialize header_.num_bytes in UnserializedMessageContext.

Bug: 847388

R=rockot@chromium.org

Change-Id: I02a661071e41a7513b4e3aa9bc929e127c78c74f
Reviewed-on: https://chromium-review.googlesource.com/1076109Reviewed-by: Ken Rockot <rockot@chromium.org>
Commit-Queue: Oliver Chang <ochang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#562591}

mojo/public/cpp/bindings/lib/unserialized_message_context.cc

@@ -15,6 +15,7 @@ UnserializedMessageContext::UnserializedMessageContext(const Tag* tag,
   header_.version = 1;
   header_.name = message_name;
   header_.flags = message_flags;
+  header_.num_bytes = sizeof(header_);
 }
 
 UnserializedMessageContext::~UnserializedMessageContext() = default;