icon: Keep symbol viewBox value when register an iconSet

#### Bug, feature request, or proposal:
Bug

#### What is the expected behavior?
When registering an SVG iconSet, every symbol may provide a custom `viewBox` attribute. This attribute should be keep in the newly SVG element created.

#### What is the current behavior?
The `viewBox` attribute is swallowed.

#### What are the steps to reproduce?

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

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
At least since beta 7

#### Is there anything else we should know?
This is related to #5188 #2981
For SVG icon set with `symbol`, see

issue ではファイルもテストも指定されていません。まず iconSet の登録と SVG シンボルの作成を追跡し、次に各シンボルのカスタム viewBox が新しく作成された SVG 要素に保持されることを確認して、その動作のカバレッジを追加してください。

索引モデルが issue の本文から書いたものです。