This is a large refactoring of the DocumentChannel process switch codepath,
with the end goal of being better able to support future process switch
requirements such as dynamic isolation on android, as well as the immediate
requirement of null principal handling.
The major changes include:
1. The logic is in C++ and has less failure cases, meaning it should be harder
for us to error out unexpectedly and not process switch.
2. Process selection decisions are more explicit, and tend to rely less on
state such as the current remoteType when possible. This makes reasoning
about where a specific load will complete easier.
3. Additional checks are made after a "WebContent" behavior is selected to
ensure that if an existing document in the same BCG is found, the load will
finish in the required content process. This should make dynamic checks such
as Android's logged-in site isolation easier to implement.
4. ProcessIsolation logging is split out from DocumentChannel so that it's
easier to log just the information related to process selection when
debugging.
5. Null result principal precursors are considered when performing process
selection.
Other uses of E10SUtils for process selection have not yet been migrated to the
new design as they have slightly different requirements. This will be done in
follow-up bugs.
Differential Revision: https://phabricator.services.mozilla.com/D120673
58c1037005