It was actress Sheila White (who was in Oliver, I Claudius and all 4 of the "Confessions of ... " films) in a clip from 1980. In "The Biograph Girl" Shelia plays the part of silent star Mary Pickford seen getting pied. This seems to be historically inaccurate because while I have several wet and mud scenes from Mary Pickford films I can find no evidence of Mary Pickford ever being pied. This is somewhat similar to the 2003 tv movie "Lucy" about the life of Lucille Ball. In the 2003 film the actress playing Lucy is seen being pied 3-4 times in different scenes from the supposed Eddie Cantor film "Roman Scandals". That is totally inaccurate because while Lucy was an extra in that film (as a Goldwyn Girl) there are no pie scenes in that film at all.
Many film biographies are inaccurate..e.g. I just recently watched the movie "Jobs" about the life of Steve Jobs and Apple, and that film is a completely watered down version of the real Steve Job's. In this sanitized film he is portrayed as uncaring person who wanders around and uses people like throwaway toilet paper....when in reality he was 10 times worse and was more like a psychopathic monster manager.