Zoom affects knob roundness

[danbeam]

Description

At certain zoom levels, the knob of a <paper-slider> is not round; it becomes an oval due to subpixel rounding.

Expected outcome

Roundness at all zoom levels

Actual outcome

Live Demo

example: https://jsfiddle.net/9j4sLw8c/ screenshots: http://imgur.com/a/PfrFT

Steps to reproduce

1. Put a paper-slider element in the page.
2. Open the page in a web browser.
3. Zoom the page.

Browsers Affected

• [X] Chrome

/cc @keanulee @frankiefu

[danbeam]

corresponding chrome bug: https://bugs.chromium.org/p/chromium/issues/detail?id=702115

[danbeam]

btw, 110% and 90% are the bad zooms