Add glue between the client and engine for Blimp

blimp/common/proto/compositor.proto

Index: blimp/common/proto/compositor.proto
diff --git a/blimp/common/proto/compositor.proto b/blimp/common/proto/compositor.proto
index bd5f826bdff89e30b627f7cfc38889ce9df6d8f9..5254f7aaec49c0da870dd71aaddc7b6ca1f7055c 100644
--- a/blimp/common/proto/compositor.proto
+++ b/blimp/common/proto/compositor.proto
@@ -13,6 +13,11 @@ option optimize_for = LITE_RUNTIME;
 package blimp;
 
 message CompositorMessage {
-  // TODO(nyquist): Include the cc proto message here.
+  // An ID that corresponds to RenderWidgetMessage.

[Kevin M, 2015/11/24 18:39:59]

Corresponds to WGM.render_widget_id

[David Trainor- moved to gerrit, 2015/11/25 17:56:42]

Done.

+  optional uint32 render_widget_id = 1;
+
+  // This represents a serialized CompositorMessage in

[Kevin M, 2015/11/24 18:39:59]

What's the reasoning behind serializing the message here? There are some nice
benefits to keeping the structure in protobuf format, e.g. less data to store
and copy around, and PB parse errors are detected earlier on and in common
codepaths. Having blimp depend on cc seems fine to me (not the other way around,
of course).

[David Trainor- moved to gerrit, 2015/11/25 17:56:42]

That would be ideal.  A few issues currently (which we can hopefully fix
eventually):
1. We have to send the data between the render and browser process, so we need
some serialized format at that point (if we use mojo eventually, we could do
this all browser side).
2. Currently it seems like Chromium doesn't support including ".proto" files in
other .proto files from another directory.  The way they're built (relative
paths, include paths, etc.) doesn't work.  We can change it, but we might have
to change every .proto file in Chromium as a part of fixing the current build
rules.

+  // cc/proto/compositor_message.proto.
+  optional bytes payload = 2;
 }