[Chrome extension] Add Referer request header if needed

[Rob--W]

This patch adds the Referer request header to PDF requests if the original PDF request included the Referer header.

Fixes #5676.

[timvandermeij]

Do you have steps on how to verify this feature? Most importantly a website that requires a referrer.

Perhaps @Hengjie could also test this?

[Rob--W]

@timvandermeij

Steps to verify:

1. Load extension: node make chromium and chromium --user-data-dir=/tmp/whatever --load-extension=build/chromium
2. Visit a page that links to a PDF where the referrer will be sent (e.g. https -> http pdf = no referrer).
   For example, https://encrypted.google.com/search?q=file%3Apdf+bitcoin.
3. Open chrome://net-internals/#events
4. Click on the link to the PDF from step 2.
5. Go back to chrome://net-internals/#events, click on the URL_REQUEST for the PDF (type "pdf" in the imput box to filter the displayed requests) and make sure that the Referer header has been added.
6. Refresh the PDF viewer and verify that the referer header is still added (using the same method as step 5).

[Rob--W]

Ping, could anyone review this?

@timvandermeij

[timvandermeij]

I'm sorry to put this on @yurydelendik's review stack, but I do not know enough about this part of the codebase to properly review this (other than testing it of course).

There does appear to be a merge conflict. Perhaps that should be resolved first?

[Rob--W]

Rebased PR.

[Hengjie]

@timvandermeij @Rob--W I can confirm it's fixed, thanks! I tested it by seting up a nginx server locally that prevented "hotlinking" by checking the referral.

[TimothyGu]

Ping.

[yurydelendik]

@Rob--W looks good, can you merge this in?

[Rob--W]

@yurydelendik Sure, I'll do this over the weekend.

[Rob--W]

Rebased and resolved trivial merge conflict (caused by an intermediate refactor of web/pdf_history.js that affected the indentation of the whole file).

Using the steps in #5823 (comment), I also verified that the patch still works in Chromium 42.0.2311.90 and 45.0.2416.0.

[yurydelendik]

Thank you for the patch.