Introduce mojo_root.

This allows projects to import generated mojo related files
by using the path relative to platform2 root directory.

BUG=None
TEST=Build locally. Trybot.

Change-Id: I152ea5b0a8c30f750e99e57628cc195e498e4a33
Reviewed-on: https://chromium-review.googlesource.com/1199782
Commit-Ready: Hidehiko Abe <hidehiko@chromium.org>
Tested-by: Hidehiko Abe <hidehiko@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
diff --git a/ml/machine_learning_service_impl.h b/ml/machine_learning_service_impl.h
index 9b188b0..9025ac4 100644
--- a/ml/machine_learning_service_impl.h
+++ b/ml/machine_learning_service_impl.h
@@ -13,7 +13,7 @@
 #include <mojo/public/cpp/bindings/binding.h>
 
 #include "ml/model_metadata.h"
-#include "mojom/machine_learning_service.mojom.h"
+#include "ml/mojom/machine_learning_service.mojom.h"
 
 namespace ml {