- Adds CodeAssembler::ConstructJS() to simplify calling JS functions as
constructors, used by NewPromiseCapability()
- Defines PromiseCapability as a special JSObject subclass, with a
non-exensible Map, and read-only non-configurable DataDescriptors which
point to its in-object fields. This allows its fields to be used by JS
builtins until there is no longer any need.

Currently, the performance benefit comes from
https://codereview.chromium.org/2567033003/, but does not appear to
regress performance in any significant way.

BUG=v8:5343
TBR=ulan@chromium.org

Review-Url: https://codereview.chromium.org/2567333002
Cr-Commit-Position: refs/heads/master@{#42014}