fix(overlay): export OverlaySizeConfig (#8932)

Author: josephperrott

src/cdk/overlay/public-api.ts

@@ -14,7 +14,7 @@ export {Overlay} from './overlay';
 export {OverlayContainer} from './overlay-container';
 export {CdkOverlayOrigin, CdkConnectedOverlay} from './overlay-directives';
 export {FullscreenOverlayContainer} from './fullscreen-overlay-container';
-export {OverlayRef} from './overlay-ref';
+export {OverlayRef, OverlaySizeConfig} from './overlay-ref';
 export {ViewportRuler} from '@angular/cdk/scrolling';
 export {ComponentType} from '@angular/cdk/portal';
 export {OverlayKeyboardDispatcher} from './keyboard/overlay-keyboard-dispatcher';