Oilpan: Implement per-size heap

This CL introduces four heaps for general type objects depending on their object sizes and thus improve memory locality.

- Objects whose size is 1 - 3 words go to the first heap.
- Objects whose size is 4 - 7 words go to the second heap.
- Objects whose size is 8 - 15 words go to the third heap.
- Objects whose size is more than 15 words go to the fourth heap.

I determined these numbers so that we can approximately evenly split general type objects observed in Gmail, Facebook, SpreadSheet, Twitter, Youtube and a couple of smoothness benchmarks.

This CL improves performance of various benchmarks:

ShadowDOM/*: ~30%
Parser/querySelector-all-*: ~10%


BUG=340522

Review URL: https://codereview.chromium.org/645373004

git-svn-id: svn://svn.chromium.org/blink/trunk@184422 bbb929c8-8fbe-4397-9dbb-9b2b20218538