#### Bug, feature request, or proposal: Request #### What is the expected behavior? `MatIconRegistry.setDefaultFontSetClass` to accept multiple css classes #### What is the current behavior? `MatIconRegistry.setDefaultFontSetClass` accepts only one class #### What is the use-case or motivation for changing an existing behavior? Some font sets require multiple classes that are tiresome to repeat on all icons. For example: `MatIconRegistry.setDefaultFontSetClasses("mdi", "mdi-48px")` would be nice to have.