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.cc b/ml/machine_learning_service_impl.cc
index 8c97447..79a7ef0 100644
--- a/ml/machine_learning_service_impl.cc
+++ b/ml/machine_learning_service_impl.cc
@@ -12,7 +12,7 @@
 #include <tensorflow/contrib/lite/model.h>
 
 #include "ml/model_impl.h"
-#include "mojom/model.mojom.h"
+#include "ml/mojom/model.mojom.h"
 
 namespace ml {