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 中没有指定文件或测试。首先跟踪 iconSet 的注册和 SVG 符号的创建，然后验证每个符号的自定义 viewBox 是否会保留在新创建的 SVG 元素中，并为该行为添加覆盖测试。

由索引模型根据 Issue 内容生成。