More navigation flags cleanup

[domenic]

I'm trying to make all the inputs to the navigation algorithm explicit. I think this is the comprehensive list:

• resource
• source browsing context (see Use of source browsing context in navigation seems totally broken to me #1130)
• navigation type ("form-submission" or "other")
• exceptionsEnabled
• replacementEnabled
• reload-triggered navigation
• entry update

I'd like to focus here on reload-triggered, entry update, and replacement enabled, as I think they can be consolidated. In particular, looking at the call sites:

• location.reload() and user-triggered reloads set reload-triggered = true, entry update = true, replacement enabled = false
• History traversal with no bfcache entry (traverse the history step 1) sets reload-triggered = false, entry update = true, replacement enabled = false
• A bunch of places use replacementEnabled, but never in combination with the other two.

So I think we should convert these three into a new parameter, call it historyHandling, with the following values. Here I use (x, y, z) to represent the values of (reload-triggered, entry update, replacement enabled).

• "default": equivalent to (false, false, false)
• "reload": equivalent to (true, true, false)
• "entry update": equivalent to (false, true, false)
• "replace": equivalent to (false, false, true)

This seems like a big improvement, since it makes it clear there are only 4 valid states, not 2³ = 8.

Any bikeshedding on these names is welcome. Example ideas:

• "reload entry update" instead of just "reload"
• "repopulate bfcache" instead of "entry update"

[annevk]

cc @mystor @smaug----