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}

• blimp/engine/browser/blimp_engine_session.cc[diff]
• blimp/engine/browser/blimp_engine_session.h[diff]
• cc/BUILD.gn[diff]
• cc/cc.gyp[diff]
• cc/proto/BUILD.gn[diff]
• cc/proto/compositor_message.proto[Added - diff]
• cc/trees/remote_proto_channel.h[Added - diff]
• content/browser/renderer_host/render_widget_host_delegate.h[diff]
• content/browser/renderer_host/render_widget_host_impl.cc[diff]
• content/browser/renderer_host/render_widget_host_impl.h[diff]
• content/browser/web_contents/web_contents_impl.cc[diff]
• content/browser/web_contents/web_contents_impl.h[diff]
• content/common/view_messages.h[diff]
• content/content_renderer.gypi[diff]
• content/public/browser/render_widget_host.h[diff]
• content/public/browser/web_contents_delegate.h[diff]
• content/renderer/BUILD.gn[diff]
• content/renderer/gpu/render_widget_compositor.cc[diff]
• content/renderer/gpu/render_widget_compositor.h[diff]
• content/renderer/render_widget.cc[diff]
• content/renderer/render_widget.h[diff]