Old code failed to walk over deleted elements, instead treating
deleted elements as "undefined" in the output array.

This is the Map equivalent of commit 2d9bfe9a.

Also micro-optimized the loops to avoid an extra call to KeyAt()
and used a direct hole comparison instead of calling IsTheHole().

R=cbruni@chromium.org
BUG=v8:4946
LOG=y

Review-Url: https://codereview.chromium.org/1965593002
Cr-Commit-Position: refs/heads/master@{#36149}