Design doc: Allow Oilpan in V8

[Michael Lippautz]

Hey all,

Here's another design doc with a proposal to allow Oilpan types to be used in V8 (internally and on its API): https://docs.google.com/document/d/1KDAHpWxjcezLR7-wnPEuugkWC4piF-bk2c8JhtqtPoE/edit?usp=sharing

tl;dr: In the future V8 Isolates will initialize with a CppHeap for Oilpan objects if none is provided during setup.

-Michael