Mojo: pack node_channel messages

Node channel messages need to be packed.

Bug: b:172869067
Change-Id: I30ac9d44347fef9e0b74267d6d09ec8589e4d3e6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2527532Reviewed-by: Ken Rockot <rockot@google.com>
Commit-Queue: Brian Geffon <bgeffon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825686}

mojo/core/node_channel.cc

@@ -45,6 +45,8 @@ enum class MessageType : uint32_t {
   BIND_BROKER_HOST,
 };
 
+#pragma pack(push, 1)
+
 struct Header {
   MessageType type;
   uint32_t padding;
@@ -130,6 +132,8 @@ struct EventMessageFromRelayData {
 };
 #endif
 
+#pragma pack(pop)
+
 template <typename DataType>
 Channel::MessagePtr CreateMessage(MessageType type,
                                   size_t payload_size,