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

在 Material 菜单概览 URL 处重现该问题，然后阅读 src/cdk/overlay/_overlay.scss 中 pointer-events 附近的内容，以及 src/lib/menu/menu-trigger.ts 中菜单滚动策略附近的内容。验证菜单打开时使用鼠标滚动会使页面移动，同时不改变报告中描述的对话框行为。

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