One of the patches in https://bugzilla.mozilla.org/show_bug.cgi?id=1202902, specifically [`Mass replace toplevel 'let' with 'var' in preparation for global lexical scope. (rs=jorendorff)`](https://hg.mozilla.org/mozilla-central/rev/380817d573cd), touches PDF.js code. Unfortunately it was landed upstream without, as far as I can tell, notifying us about it.
This patch uplifts the relevant changes to avoid future merge conflicts, and for consistency also tweaks `PdfJs-stub.jsm`.
This patch removes the only remaining CPOW usage from the code-base, and should thus fix https://bugzilla.mozilla.org/show_bug.cgi?id=1071082.
*Note:* This will not fix the "Open With Different Viewer" button in e10s, since clicking it still fails with `frontWindow is null` in the console, but I do believe that that issue is somewhat orthogonal to the current patch.
