Previously, cases like
  var [foo]
led to a parser crash because the parser tried to do something with
the initializer, which was not syntactically present.

This patch fixes the parser issue (implicitly creating an undefined
initializer) and inserts a check for array destructuring that the
right-hand side is coercible to an object, so it can have iterator
methods called on it safely.

BUG=v8:4462
LOG=Y
R=adamk

Review URL: https://codereview.chromium.org/1384413002

Cr-Commit-Position: refs/heads/master@{#31128}