A bit was added in the context length slot to indicate if
the context had an extension slot. It turns out that
we need this information much earlier and so this flag is now
in the scope info instead.

This CL removes this bit from length, since it was not
used anymore.

I also renamed HasContextExtension to HasContextExtensionSlot
to differentiate from Context::has_extension which returns
true only if the context has an extension slot and the
extension is not the undefined object.

Bug: v8:9744
Change-Id: I7c37105b7afed34e8f480a64596fab285388f21b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1879935
Commit-Queue: Victor Gomes <victorgomes@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64577}