gpu_channel.{h|cc} is doing a lot and has three major classes: GpuChannel, GpuChannelMessageFilter, and GpuChannelMessageQueue.

In the interest of clarifying the flow of messages, I'd like to dice these up into separate files.

I think this will also make it easier for us to have the conversation about what the roles of these classes might be with Mojo IPC in the future.

For example, GpuChannelMessageQueue is a deque of GpuChannelMessage which holds an IPC::Message. Maybe we can make that a callback instead as well within this bug (or a separate bug)?