Fix Polymer UI broken in newest Chrome due to ::content -> /content/ change

The advanced CSS rule notation has been changed in the recent Chrome to match the recent change in W3C specification: in particular, ::content pseudo-class has been removed, and /content/ combinator introduced to replace it.

Unfortunately, combining the new and the old syntax via a comma doesn't work:

::content > *, * /content/ * { ... }

so all affected rules had to be fully duplicated.
Undo some unintended changes
Merge pull request #1375 from ussuri/fix-polymer-ui-broken-in-new-chrome

Fix polymer ui broken in new chrome