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?

まず Angular Material のスタイルで `.mat-drawer-container` と `md-drawer-container` コンポーネントのエントリポイントを検索し、その後、標準とは異なるアプリの背景と drawer を使って問題を再現します。drawer の動作を壊すことなく、アプリの背景が drawer ページのコンテンツの背後に表示されたままになり、その領域にカバレッジがある場合は関連するテストが更新または追加されていれば完了です。

索引モデルが issue の本文から書いたものです。