fix(material/select): add 8px margin if it gets close to viewport

Author: andrewseguin

src/material/select/select.html

@@ -40,6 +40,7 @@
   [cdkConnectedOverlayPositions]="_positions"
   [cdkConnectedOverlayWidth]="_overlayWidth"
   [cdkConnectedOverlayFlexibleDimensions]="true"
+  [cdkConnectedOverlayViewportMargin]="8"
   (backdropClick)="close()"
   (overlayKeydown)="_handleOverlayKeydown($event)">
   <div