That way, we don't need to create a context-less constructor function.
Instead, we use the constructor_or_backpointer (or null) field, and
rename it to constructor_or_fti_or_backpointer so nobody is confused.

Note that technically, we still have JSFunctions without contexts, as
they're temporarily created in the deoptimizer.

BUG=v8:6084
R=dcheng@chromium.org,bmeurer@chromium.org,verwaest@chromium.org

Change-Id: I084f052533c317f2cbfb9c35e1acf40263c6257b
Reviewed-on: https://chromium-review.googlesource.com/454636
Commit-Queue: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43834}