Fixes #168, scrollbutton glitch, tests added

[felvhage]

This fixes #168. When using Browser-Zoom scrollLeft can be a fractional number. Using Math.ceil on scrollLeft ensures that the condition will be met properly.

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[felvhage]

I fixed the tests, so they work in shady & shadow dom-mode. I have tested Chrome, IE11 and Firefox locally.
The Travis CI build however, is still not happy with me. I read something is wrong in Firefox (?), but i cannot reproduce the problem locally.

[bicknellr]

This needs to be wrapped in something like

if (Object.keys(document.createElement('div').style).indexOf('zoom') >= 0) {

so that it doesn't attempt these if the browser doesn't support zoom.

[bicknellr]

reminder :)

[bicknellr]

LGTM, except for the comment I left about testing if zoom is supported. Also, I think you've won the test-to-code ratio award for this PR!

[felvhage]

Thank you. You just gave me the motivation, to try some more stuff out ;-)

[bicknellr]

I meant that the test suite itself should be wrapped. You don't actually need to remove them but, if the browser doesn't support the zoom property in the first place, they should probably be ignored since the tests rely on it. Put another way, isn't the 'zoomed, scrollable paper-tabs' suite now the same as one of the tests in the first suite?

[felvhage]

Yes, the test is now the same, as the other ones. I'm trying to exclude piece, by piece to track down the problem, because i cannot reproduce it locally. Currently, there is no use of the zoom-property at all. Perhaps there is some structural problem. I'll try some more.

[bicknellr]

Oh! This isn't a problem with your code: Firefox and our test setup aren't playing well together right now. Don't worry about making the Firefox CI tests work. Until we get the underlying issue resolved, it's fine as long as it passes locally.

[felvhage]

Yes, that would have been my final conclusion aswell. I'll put it back together then. :-)

[felvhage]

Should be back to where i started again.

[bicknellr]

(https://github.com/PolymerElements/paper-tabs/pull/169/files#r66304803)