When Firefox is run in e10s mode, which will soon be the default, the PDF.js zoom dropdown menu doesn't look right. This is apparently because the `<select>` DOM element is rendered in the parent, and that all the necessary style information isn't sent up from the child. See the discussion in https://bugzilla.mozilla.org/show_bug.cgi?id=910022.
Besides this causing the PDF.js UI to *look* worse in e10s, notably it also means that the `customScaleOption` isn't hidden like it ought to be.
To work-around that, this patch utilizes the `hidden` attribute, since https://bugzilla.mozilla.org/show_bug.cgi?id=1242450 at least made that work in e10s.
Fixes https://bugzilla.mozilla.org/show_bug.cgi?id=1194700.
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