This class is used by both parent and child processes. It's more heavily used on the parent vs the child side.
With this change, even though the PermissionManager class should itself be now thread-safe, I have still
kept assertions on most of it's public methods for main thread in place because I wanted to keep the regression
risk minimal with this change while still satisfying my criteria about allowing some operations like adding new
and reading existing permissions thread-safe such that they can be performed off-main thread.
There might be some permission manager callers here which previously needed to switch to main thread in order to
invoke operations; they would no longer need to do that. After gaining some confidence with this limited exposure
maybe we can slowly start looking into the callers and see if they can benefit from this thread-safety now i.e.
if they do not need to switch to main thread just for the sake of PermissionManager usage.
Since, this is a singleton class; ofcourse, the creation, initialization, etc. are still main thread only. Also,
any DB operation can also only be performed on the corresponding thread only.
Differential Revision: https://phabricator.services.mozilla.com/D244608
479b5b0f5a