[Tooltip] Font size for tooltip should be set from typography config instead of hardcoded

[jiefenghe]

What is the expected behavior?

https://github.com/angular/material2/blob/master/src/lib/tooltip/_tooltip-theme.scss
Font size should come from typography config https://github.com/angular/material2/tree/master/src/lib/core/typography.

What is the current behavior?

Font size is hardcoded in tooltip theming file.
https://github.com/angular/material2/blob/master/src/lib/tooltip/_tooltip-theme.scss

What are the steps to reproduce?

Providing a StackBlitz reproduction is the best way to share your issue.

StackBlitz starter: https://goo.gl/wwnhMV

Which versions of Angular, Material, OS, TypeScript, browsers are affected?

Is there anything else we should know?