Simplifies the implementation of IteratorClose in IteratorBuiltinsAssembler, and makes clear that it is only invoked when an exception occurs. Adds exception handling support to GetIterator, IteratorStep, and IteratorCloseOnException.

Moves the Promise.all resolveElement closure and it's caller to
builtins-promise-gen.cc.

Instead of creating an internal array (and copying its elements into a
result
array), a single JSArray is allocated, and appended with
BuildAppendJSArray(),
falling back to %CreateDataProperty(), and elements are updated in the
resolve
closure the same way. This should always be unobservable.

This CL increases the size of snapshot_blob.bin on an x64.release build
by 8.51kb

BUG=v8:5343
R=cbruni@chromium.org, gsathysa@chromium.org, jgruber@chromium.org, hpayer@chromium.org, tebbi@chromium.org

Change-Id: I29c4a529154ef49ad65555ce6ddc2c5b7c9de6b3
Reviewed-on: https://chromium-review.googlesource.com/508473
Commit-Queue: Caitlin Potter <caitp@igalia.com>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45946}