Arrow keys move slider and scroll the page

[stevenjb]

Description

Pressing up/down/left/right will change the slider value but will also scroll the screen.

Expected outcome

The page should not scroll. Other elements (e.g. up/down in paper-listbox) do not do this.

Actual outcome

The page scrolls (when scrollable, e.g. when the window is small).

Live Demo

This can be seen by making the browser window for the paper-slider demo page small: https://elements.polymer-project.org/elements/paper-slider?view=demo:demo/index.html&active=paper-slider

[stevenjb]

Apologies for all of the churn above, but I believe that pull request #165 is correct (or at least works for me).