Stub out Firefox addon Telemetry wrapper

[georgf]

We are planning to remove the addon histogram APIs from Firefox
Telemetry in bug 1353295.

pdf.js is one of 3 projects that ever used them.
The easy solution here seems to just stub out all calls that use addon histograms, that should keep things working just as before.

linting passes fine locally.
I had trouble getting the tests to run due to grunt makeref running into md5 mismatches.

[georgf]

@yurydelendik for review?

[Snuffleupagus]

With these changes, I think that you can also remove the following lines since they shouldn't be neecessary: https://github.com/mozilla/pdf.js/blob/master/extensions/firefox/content/PdfJsTelemetry-addon.jsm#L15-L16.

I had trouble getting the tests to run due to grunt makeref running into md5 mismatches.

Unfortunately, running e.g. gulp test won't help with testing the add-on code.
You can however use gulp firefox to build the add-on locally and then install that in Firefox (pleases note that xpinstall.signatures.required needs to be turned off, since a locally built add-on won't be signed).

[yurydelendik]

Thank you for the patch.

[timvandermeij]

Is there a reason why we cannot just remove the file altogether? I don't really see why we need to keep this around if it does nothing anymore, especially since the add-on is primarily used for development purposes. I don't think we need to be backwards compatible here, unless I'm missing something...

[yurydelendik]

We use it at PdfStreamConverter.jsm, we can further rename it to have a "-stub", similar to pdfjs-stub.jsm

[timvandermeij]

Thank you. In that case we probably need to keep it around for compatibility. Renaming it to have a -stub suffix, together with removing https://github.com/mozilla/pdf.js/blob/master/extensions/firefox/content/PdfJsTelemetry-addon.jsm#L15-L16, would be good, but it's also fine as-is then.