- Remove weak-collection.js
- Adds TFJ builtins for WeakSet and WeakMap constructors
- Unified helpers and constructor behavior into a BaseCollectionsAssembler
- Fast paths for...
  - unmodified constructor function
  - argument is a fast JS array
  - entries are fast JS arrays, for Map/WeakMap
  - no arguments passed

Quick benchmarks shows significant improvements (1.12x - 5.7x!) for ALL collection constructors (weak and non-weak):
https://github.com/peterwmwong/v8-perf/blob/master/weakcollection-constructor/README.md

More could be done for performance.  Currently we always call out to JS to add entries, if we knew the prototype was unmodified, we could call the builtins directly.

Bug: v8:5049, v8:6604
Change-Id: Id7912c1eed5bcf512df7fd6238f04166a8a5937e
Reviewed-on: https://chromium-review.googlesource.com/760385Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49343}