Miscellaneous e10s fixes

The *relevant* commit message already said: `Update the license header in extensions/firefox/content/pdfjschildbootstrap.js`, so I'm not really sure how this can be considered dubious.
"Miscellaneous e10s fixes" are the title of the PR, *not* the indiviual commits. Anyway, I've amended the *commit* with a link to the relevant Bugzilla comment.

The PDF.js project uses the Apache v2 license, but unfortunately the file didn't land with the correct license header. Hence I figured I would address that in this PR.
Since I needed to submit a few other e10s fixes, in this case, it seemed reasonable to submit a few different *commits* in the same PR to speed up the merging process.