<md-menu> shrinks on iOS safari

[echenley]

If an md-menu is opened when iOS safari is in "minimal UI" mode (after scrolling down), it will shrink to a tiny size. Example:

This doesn't affect the demo site since the behavior is prevented by restricting overflow scrolling to a div, rather than the body element.

[brad-sobie]

+1

[ThomasBurleson]

@robertmesserle - can you review and test this?