27cb632c76
This adds extra headers and query parameters to identify MAR GET requests as originating from browsing profiles or background task profiles. This extracts a simple existing function for setting headers and then makes it available to `nsIIncrementalDownload`. The existing function is not the most pleasant API, but it exists and is much easier than working through the details of storing header name-value pairs until they are ready to be used. It's also the same API exposed by Windows BITS, so there's no additional manipulation required to support BITS. Differential Revision: https://phabricator.services.mozilla.com/D221860