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Follow-up to https://chromium-review.googlesource.com/576183 ("bindings:
Return the right property descriptors for legacy platform objects"), which
introduced the indexed descriptor callbacks for bindings.

Even if a given IDL (such as Storage.idl) only supports named properties,
our bindings layer installs a handler for indexed properties that, in this
case, mostly just ends up falling back to the named property handlers.

However, we were not defining a descriptor callback for indexed properties
when the IDL only supports named properties. Consequently, we were not
setting the right property descriptor values for a given numeric property,
leading to bizarre cases where, e.g. if Storage only had numeric keys,
Object.values() and Object.keys() would return arrays of diffferent
lengths (the latter would be an empty one, actually).

This change is a bit complicated because we install our indexed and named
property handlers differently: the former is created with descriptor and
definer callbacks, whereas the latter uses the query callback signature
instead. This means the indexed property descriptor callback for named
properties needs to call the query callback and translate its return value
into a format suitable for descriptor callbacks, which is not very
efficient. Ideally, both named and indexed properties should converge in
terms of how their handlers are declared.

Despite the fact that this is a follow-up commit, the original CL did not
introduce a regression; interfaces that only support named properties were
just behaving like they did in the past, with an indexed descriptor callback
not being set.

Bug: 719703, 764633
Change-Id: I1ebaabe61f5f8b6cadb0cfa8b41664a64e49e51b
Reviewed-on: https://chromium-review.googlesource.com/660497
Commit-Queue: Raphael Kubo da Costa (rakuco) <raphael.kubo.da.costa@intel.com>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Reviewed-by: Hitoshi Yoshida <peria@chromium.org>
Reviewed-by: Kenichi Ishibashi <bashi@chromium.org>
Reviewed-by: Joshua Bell <jsbell@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#501591}