Originally, only BindingIdentifiers were a legal operand for the `...` ellipsis
in a function rest parameter. This has since changed, allowing the rest array
to be destructured.

The grammar is now the following:

```
FunctionRestParameter[Yield]:
    BindingRestElement[?Yield]

BindingRestElement[Yield]:
    ... BindingIdentifier[?Yield]
    ... BindingPattern[?Yield]
```

*Spec change: https://github.com/tc39/ecma262/commit/d322357e6be95bc4bd3e03f5944a736aac55fa50
*TC39 Discussion: https://github.com/tc39/tc39-notes/blob/master/es7/2015-07/july-28.md#66-bindingrestelement-should-allow-a-bindingpattern-ala-assignmentrestelement

BUG=v8:4627, v8:2159
LOG=N
R=littledan@chromium.org, adamk@chromium.org, wingo@igalia.com, rossberg@chromium.org

Review URL: https://codereview.chromium.org/1532873004

Cr-Commit-Position: refs/heads/master@{#33192}