fix(material/icon): style icons properly when within a MDC button

Author: annieyw

src/material/icon/icon.ts

@@ -250,6 +250,11 @@ export class MatIcon extends _MatIconMixinBase implements OnInit, AfterViewCheck
     // Update font classes because ngOnChanges won't be called if none of the inputs are present,
     // e.g. <mat-icon>arrow</mat-icon> In this case we need to add a CSS class for the default font.
     this._updateFontIconClasses();
+
+    // Add appropriate MDC class if it is inside a MDC button to be styled properly
+    if (this._elementRef.nativeElement.parentElement?.classList.contains('mdc-button')) {
+      this._elementRef.nativeElement.classList.add('mdc-button__icon');
+    }
   }
 
   ngAfterViewChecked() {