[FloatingActionButton] Fix thin white border

[ludoviccyril]

• PR has tests / docs demo, and is linted.
• Commit and PR titles begin with [ComponentName], and are in imperative form: "[Component] Fix leaky abstraction".
• Description explains the issue / use-case resolved, and auto-closes the related issue(s) (http://tr.im/vFqem).

Hello there.

I've noticed that there is a thin white border around FloatingActionButton's. It's particularly visible when the FloatingActionButton and the underlying element use dark colors, and it really looks bad:

The expected look and feel is:

Upon taking a look at the source code, I realized this happens because FloatingActionButton's contain a Paper element, whose default color is white. I suggest setting the background-color of that Paper element to transparent to solve this issue.

Here's the WebpackBin for this issue.

[oliviertassinari]

I confirm the issue. That looks good to me. Thanks.