With the old process selector service implementation, non-cached loads
would skip the call into the process selector.
This implementation piggybacks atop the existing 'redirectTo' mechanism.
This is unfortunately not the perfect system for catching these loads,
as it doesn't provide an opportunity for performing a final process
switch before redirecting to a non-http channel. In addition, it doesn't
provide indication that a redirect is going to happen, causing
potentially unnecessary process switches.
Not all places where 'redirectTo' is supported use this mechanism. This
process switching mechanism is only checked in situations after
http-on-examine-response.
Potential future changes include:
1. Moving these checks closer to the real 'OnStartRequest' call (e.g.
in ContinueProcessNormal + ContinueOnStartRequest3). This would mean
that loads other than the final load will not cause process swaps.
2. Adding a callback before a redirect is performed, passing in the new
channel, and allowing modifications to be made. This would allow
performing a process swap before redirecting to a non-http(s)
channel.
Depends on D15609
Differential Revision: https://phabricator.services.mozilla.com/D15610
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