This fixes a missing fast-path check in the code-stub implementation of
the {Array.prototype.filter} method. Appending to the target JSArray is
only correct if the underlying length did not change.

R=jgruber@chromium.org
TEST=mjsunit/regress/regress-6657
BUG=v8:6657

Change-Id: Ida8d3511485b649b70d9a4b161742d494ebe4dac
Reviewed-on: https://chromium-review.googlesource.com/600467Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47156}