Click/enter on controls inside a checkable widget shouldn't check the widget

[mgiuffrida]

https://jsfiddle.net/u5m3zgxx/1/

In Chrome, clicking (or pressing Enter) on a <button> or <a> inside of a <label> for an <input> does not check the corresponding <input>.

Elements like paper-checkbox and paper-radio-button do not behavior this way; tapping the button will check the checkbox/radio-button.

Just using a <label> around a <paper-checkbox> instead of putting the content directly inside the <paper-checkbox>'s light DOM isn't great because the styling is different.