Open menu should support background scrolling

#### Bug, feature request, or proposal:
Bug? Reading the source, when having a menu open, I believe the background should be able to be scrolled using the mouse. The background can be scrolled with the arrow keys.

#### What is the expected behavior?
The background can scroll.

#### What is the current behavior?
The background cannot scroll.

#### What are the steps to reproduce?

1. Visit https://material.angular.io/components/menu/overview
2. Open the menu by clicking the trigger: "Menu"
3. Attempt to scroll the page (not the menu) using the mouse.

#### What is the use-case or motivation for changing an existing behavior?
A dialog box should block background scrolling, but the menu should not.

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
Current

#### Is there anything else we should know?
It is blocked by `pointer-events: auto`: https://github.com/angular/material2/blob/4d82f833c7e4a22814b8e225f05ef4ba01a86ad6/src/cdk/overlay/_overlay.scss#L61.
The menu does have the scroll strategy of reposition, which I believe should mean that the background should be able to be scrolled: https://github.com/angular/material2/blob/fe37cb2551929844d5467e1b9bf45032df35981f/src/lib/menu/menu-trigger.ts#L57

Reproduziere das Problem unter der URL zur Übersicht des Material-Menüs und lies dann src/cdk/overlay/_overlay.scss im Bereich von pointer-events sowie src/lib/menu/menu-trigger.ts im Bereich der Scroll-Strategie des Menüs. Überprüfe, dass das Scrollen mit der Maus die Seite bewegt, während ein Menü geöffnet ist, ohne das im Bericht beschriebene Dialogverhalten zu ändern.

Vom Indexierungsmodell aus dem Issue-Text verfasst.