blimp: Add mode to use the updated compositing path on the client.

blimp/client/core/switches/blimp_client_switches.h

Index: blimp/client/core/switches/blimp_client_switches.h
diff --git a/blimp/client/core/switches/blimp_client_switches.h b/blimp/client/core/switches/blimp_client_switches.h
index 9100628b76b486d41af5eaac9efebfbaed609d3c..819de3376abdade6e090bcb18053d0fb8339c019 100644
--- a/blimp/client/core/switches/blimp_client_switches.h
+++ b/blimp/client/core/switches/blimp_client_switches.h
@@ -33,6 +33,10 @@ extern const char kEngineTransport[];
 // Enables downloading the complete page from the engine.
 extern const char kDownloadWholeDocument[];
 
+// Enables the updated compositing path, which uses a threaded
+// LayerTreeHostInProcess for the BlimpCompositor.
+extern const char kEnableUpdatedCompositingPath[];
+
 }  // namespace switches
 }  // namespace blimp