Preload the font used by `.keyboard-focus`

[blasten]

If a button receives focus from the keyboard, the font-face might not be ready (downloaded & decoded). This creates an undesirable visual effect; however, if the font-face is requested ahead in time, we can avoid that effect.

This seems to be the issue in https://github.com/PolymerElements/paper-button/issues/24 \cc @frankiefu

[cdata]

Does this actually work?

Also, maybe you should run the tests again?

Also also, sorry for responding to this so late.

Finally :+1:

[cdata]

@blasten Can you rebase this branch, and then reopen the PR? Thanks.