Prepare SyntheticPointerAction to handle mouse actions

content/common/input/synthetic_pointer_action_params.h

Index: content/common/input/synthetic_pointer_action_params.h
diff --git a/content/common/input/synthetic_pointer_action_params.h b/content/common/input/synthetic_pointer_action_params.h
index 641787c379ebdecf6e2432eba8e9ad6171ac9183..4e51f036b7352174c7fa2573d0d9b0cd090da414 100644
--- a/content/common/input/synthetic_pointer_action_params.h
+++ b/content/common/input/synthetic_pointer_action_params.h
@@ -51,6 +51,8 @@ struct CONTENT_EXPORT SyntheticPointerActionParams
   void set_index(int index) {
     DCHECK(pointer_action_type_ != PointerActionType::PROCESS &&
            pointer_action_type_ != PointerActionType::FINISH);
+    // For all mouse pointer actions, the index should always be 0.
+    DCHECK(gesture_source_type != MOUSE_INPUT || index == 0);
     index_ = index;
   }
 
@@ -65,6 +67,7 @@ struct CONTENT_EXPORT SyntheticPointerActionParams
   int index() const {
     DCHECK(pointer_action_type_ != PointerActionType::PROCESS &&
            pointer_action_type_ != PointerActionType::FINISH);
+    DCHECK(gesture_source_type != MOUSE_INPUT || index_ == 0);
     return index_;
   }