Popover should default to use anchorEl's parent body if container prop isn't provided

[ianschmitz]

• I have searched the issues of this repository and believe that this is not a duplicate.

Expected Behavior

Popover renders within the context of the anchorEl document.

Current Behavior

Popover will render in the global document unless an explicit container prop is passed to the popover

Steps to Reproduce (for bugs)

See https://codesandbox.io/s/6lqo45vzvk which is a demo I setup for #10037, which includes an explicit container prop (which is clunky).

Context

The changes made in #9613 have been a great step forward in the right direction, and have largely fixed the issues I was having with rendering MUI components in a separate window (many thanks!!! 🎉). However any component that relies on Popover still has issues, as the container used when creating the Portal defaults to the global document.body object, instead of respecting the document.body containing the anchorEl prop.

Your Environment

Tech	Version
Material-UI	1.0.0-beta.30
React	16
browser	all
etc