Load file:-URLs in the main thread.

[Rob--W]

QtWebKit does not support file:-URLs in Web Workers.
To solve this issue, I'm loading file:-URLs in the main thread for generic builds. file:-URLs load relatively quick, so there is no need for sophisticated load progress notifications.

Fixes #5057

[Rob--W]

@timvandermeij Could you take a look at this PR?
For background, see the test case at #5057 (comment) and the subsequent comment (#5057 (comment)).

[timvandermeij]

I think this looks good, but I'll let someone else make the call for this one. One comment though: can we rename x to something more meaningful, i.e., request or something like that?

[Rob--W]

I've renamed x to xhr (which is consistent with similar code in src/core/network.js).

[yurydelendik]

I guess we can take it. I think we need to minimize commented code for linting. Can you move xhr logic into e.g. file_utils.js and wrap into that function that will return Promise, for example readLocalFile(file).then(function () { open(); }, function() { error() });. (In the future we can move 'data' base64 decoding stuff there as well)

It makes sense to have PDFJS.getUrlProtocol(file) === 'file' instead of file && file.lastIndexOf('file:', 0) === 0 (I noticed we are using this operation a lot across the project, e.g. in utils.js or chromecom.js)

[Rob--W]

@yurydelendik I have removed the comments to allow linting. This snippet will be included in the GENERIC build only, which is the default, so it makes sense to have the code uncommented by default.

I haven't added getUrlProtocol because the uses in the viewer code (chromecom.js and this PR) are already obvious and very readable.

[yurydelendik]

Could you move this line after xhr.open to avoid exceptions in Firefox?

[Rob--W]

Done.

[yurydelendik]

Thanks