Ignore soft message size limits on Serialize calls

Application code may call mojom structs' generated Serialize() method to
produce a local array of bytes representing the serialized object. These
methods ultimately use the same message serialization logic that applies
to IPC messages.

Soft size limit checks are done during the serialization process, and so
manual Serialize calls can still trigger crash dumps when serializing
very large objects.

Since the intent of the checks is to reveal oversized IPC messages for
performance reasons, there's no reason to apply them on these Serialize
calls.

This CL modifies the common path used by all generated Serialize()
methods to ensure that the size checks are bypassed.

Bug: 1141987
Change-Id: I936ba58cf682c4b63310f746cd19a933c82a70b2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2495835Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/master@{#821060}

mojo/public/cpp/bindings/lib/serialization.h

@@ -63,7 +63,11 @@ struct MojomSerializationImplTraits<
 template <typename MojomType, typename UserType>
 mojo::Message SerializeAsMessageImpl(UserType* input) {
   SerializationContext context;
-  mojo::Message message(0, 0, 0, 0, nullptr);
+  // Note that this is only called by application code serializing a structure
+  // manually (e.g. for storage). As such we don't want Mojo's soft message size
+  // limits to be applied.
+  mojo::Message message(0, 0, 0, 0, MOJO_CREATE_MESSAGE_FLAG_UNLIMITED_SIZE,
+                        nullptr);
   typename MojomTypeTraits<MojomType>::Data::BufferWriter writer;
   MojomSerializationImplTraits<MojomType>::Serialize(
       *input, message.payload_buffer(), &writer, &context);