When a remote type mismatch is found for a subframe, this patch checks if
fission is enabled for that window. If it is, it triggers a process switch,
continuing the load in a new process.
With this patch, subframes will only change process when navigating to a HTML
subframe, and not when navigating to a non-HTML subframe. That will be fixed in
a follow-up. This patch also does not change the remote type selection logic,
so only very limited types of remote iframes are supported.
Differential Revision: https://phabricator.services.mozilla.com/D27513
3ee1e26b6d