- Add WeakMapPrototypeSet and WeakSetPrototypeAdd TFJ builtins
  - Fast paths for...
    1) existing key
    2) new key when ObjectHashTable has a "sufficient capacity"
- Create WeakCollectionsBuiltinsAssembler to consolidate common WeakMap/WeakSet code generation
- Convert existing WeakMapLookupHashIndex to use WeakCollectionsBuiltinsAssembler

Some quick benchmarks shows performance gains of...
- 1.56x - 1.98x for WeakMap constructor
- 1.66x - 2.06x for WeakSet constructor
- 1.50x - 2.11x for WeakMap.p.set
- 1.54x - 2.26x for WeakSet.p.add

https: //github.com/peterwmwong/v8-perf/blob/master/weakcollection-set/README.md
Bug: v8:5049, v8:6604
Change-Id: I3499d46be6b2b3b1d8d46720ebe86cc5142ee542
Reviewed-on: https://chromium-review.googlesource.com/737935
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49036}