The "writable" property descriptor may legally change during the call to
AnythingToArrayLength(). This change needs to be honoured before calling
JSArray::SetLength(). The change is only honoured when the "length"
property was previously writable, so that changes during a call to
DefineOwnPropertyIgnoreAttributes() is ignored.

BUG=v8:5688
R=cbruni@chromium.org, verwaest@chromium.org, jkummerow@chromium.org

Review-Url: https://codereview.chromium.org/2543553002
Cr-Commit-Position: refs/heads/master@{#41396}