Add IPC messages to transfer compositor protos

- Be able to send and receive compositor protos to/from the render
  process and the browser process.  Messages are piped through the
  browser process to/from the network and renderer.  A separate client
  (go/blimp) will render the compositor protos.  This is part of the
  compositor refactor around splitting ThreadProxy into two components.
- Hook the proto sending into the BlimpEngineSession class.
- Set up a RemoteProtoChannel interface on the CC side to be used by the
  ChannelMain class once we start building that from the LayerTreeHost.

BUG=552055
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1442853005

Cr-Commit-Position: refs/heads/master@{#360452}

diff --git a/cc/BUILD.gn b/cc/BUILD.gn
index 998223c..c3b9b8a 100644
--- a/cc/BUILD.gn
+++ b/cc/BUILD.gn

@@ -493,6 +493,7 @@
     "trees/proxy_common.h",
     "trees/proxy_impl.h",
     "trees/proxy_main.h",
+    "trees/remote_proto_channel.h",
     "trees/scoped_abort_remaining_swap_promises.h",
     "trees/single_thread_proxy.cc",
     "trees/single_thread_proxy.h",