mus: Allow embedder to intercept events.

components/mus/public/interfaces/window_tree_constants.mojom

Index: components/mus/public/interfaces/window_tree_constants.mojom
diff --git a/components/mus/public/interfaces/window_tree_constants.mojom b/components/mus/public/interfaces/window_tree_constants.mojom
index a4540aa0e5b35e00ab2d6b890443d30229753166..c8e1c689510d1f780813e2169f8aed74746f8c35 100644
--- a/components/mus/public/interfaces/window_tree_constants.mojom
+++ b/components/mus/public/interfaces/window_tree_constants.mojom
@@ -47,3 +47,11 @@ enum EventResult {
   UNHANDLED,
 };
 
+// When this flag is set in a call to Embed(), the embedder (i.e. the client
+// that is making the call to Embed()) will receive events that are targeted to
+// the embedded client. The embedded client will not receive any input events
+// from the window server. However, the embedder can choose to dispatch events
+// to the embedded client through other mechanism.
+// TODO(sad): Provide an API in mus for the embedder to dispatch events to the
+// embedded client. https://crbug.com/621085
+const uint32 kEmbedFlagEmbedderInterceptsEvents = 0x01;