In order to restore an element's 'Constrain to Massing' property to 'Dependent', it must be in the main model with the massing. 
   Namespace:   Autodesk.Revit.DB  
  Assembly:   RevitAPI  (in RevitAPI.dll) Version: 2015.0.0.0 (2015.0.0.0) 
Syntax
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