Add the basic infrastructure for the Behavior feature type: BehaviorFeature and

extensions/common/features/simple_feature.h

Index: extensions/common/features/simple_feature.h
diff --git a/extensions/common/features/simple_feature.h b/extensions/common/features/simple_feature.h
index 27213de09eed55a9afcba7ef3186fa11fe0e359a..dadf7a6371f503cbdc013fb80e4b3cf8a0e35130 100644
--- a/extensions/common/features/simple_feature.h
+++ b/extensions/common/features/simple_feature.h
@@ -27,8 +27,7 @@ class SimpleFeature : public Feature {
   ~SimpleFeature() override;
 
   // Similar to Manifest::Location, these are the classes of locations
-  // supported in feature files; "component" implies
-  // COMPONENT/EXTERNAL_COMPONENT manifest location types, etc.
+  // supported in feature files.
   //
   // This is only public for testing. Production code should never access it,
   // nor should it really have any reason to access the SimpleFeature class
@@ -36,6 +35,7 @@ class SimpleFeature : public Feature {
   enum Location {
     UNSPECIFIED_LOCATION,
     COMPONENT_LOCATION,
+    EXTERNAL_COMPONENT_LOCATION,
     POLICY_LOCATION,
   };