Proposal: remove background-color from .mat-drawer-container

#### Bug, feature request, or proposal:
Proposal: remove `background-color` from `.mat-drawer-container`

#### What is the expected behavior?
The common background that I've set in my app appears on a page with a drawer

#### What is the current behavior?
Putting a drawer on a page requires putting the page contents inside `md-drawer-container` which comes with `background-color` set that overrides any app background that's been set.

#### What are the steps to reproduce?
Add a drawer to a page with a non-standard background

#### What is the use-case or motivation for changing an existing behavior?
Overlays for drawers and sliders are typically supposed to block the majority of the UI. IMHO material shouldn't be providing a background-color for this majority.

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
2.0.0-beta.10

#### Is there anything else we should know?

Empieza buscando `.mat-drawer-container` en los estilos de Angular Material y el punto de entrada del componente `md-drawer-container`; después, reproduce el problema con un fondo de aplicación no estándar y un drawer. Se considera terminado cuando el fondo de la aplicación sigue siendo visible detrás del contenido de la página del drawer sin romper el comportamiento del drawer, y se han actualizado o añadido las pruebas pertinentes si esa área tiene cobertura.

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