ES6 specs the function length property (it was not part of ES5) and
it makes it configurable.

BUG=v8:3045
LOG=N
R=mstarzinger@chromium.org, adamk@chromium.org
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:linux_chromium_rel_ng;tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/993073002

Cr-Commit-Position: refs/heads/master@{#27190}