provide method to get _backdropElement in OverlayRef

[vthinkxie]

Bug, feature request, or proposal:

feature request

What is the expected behavior?

provide method to get private member _backdropElement in OverlayRef

What is the current behavior?

there is no way to get backdropElement in OverlayRef

What is the use-case or motivation for changing an existing behavior?

I want to subscribe more events like contextmenu or double click on the backdropElement, but the OverlayRef only provide overlayElement here
https://github.com/angular/material2/blob/a48f75b454042798da0a94cfc62e0d94b26be7e1/src/cdk/overlay/overlay-ref.ts#L51
and backdropClick here
https://github.com/angular/material2/blob/a48f75b454042798da0a94cfc62e0d94b26be7e1/src/cdk/overlay/overlay-ref.ts#L184

can OverlayRef provide an method to access the _backdropElement at
https://github.com/angular/material2/blob/a48f75b454042798da0a94cfc62e0d94b26be7e1/src/cdk/overlay/overlay-ref.ts#L29
thanks very much

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