Add the plumbing that will carry a clipboard item from a chromoting client to a host.

remoting/proto/event.proto

Index: remoting/proto/event.proto
diff --git a/remoting/proto/event.proto b/remoting/proto/event.proto
index dcc2aaebea7d2dfeeb67fcca54608612886f631c..e42f5d3bc51b61568205ea57e0b5ae846b1513c9 100644
--- a/remoting/proto/event.proto
+++ b/remoting/proto/event.proto
@@ -48,3 +48,13 @@ message MouseEvent {
   optional MouseButton button = 5;
   optional bool button_down = 6;
 }
+
+// Defines an event that sends clipboard data between peers.
+message ClipboardEvent {
+
+  // The MIME type of the data being sent.
+  optional string mime_type = 1;
+
+  // The data being sent.
+  optional bytes data = 2;
+}