In ES6, the TypedArray constructor can be called either with an
array-like object or an iterable. The code previously handled
only array-like objects. This patch switches to supporting
iterables while throwing in an optimization to make Arrays
get allocated the old way, without an extra copy.

BUG=v8:4090
LOG=Y
R=adamk

Review URL: https://codereview.chromium.org/1181903003

Cr-Commit-Position: refs/heads/master@{#29031}