Decrease regular heap object size to 400k. In a follow up, we can now get rid of
the new space border page while keeping the 1M minimum new space size.

This reverts commit 1617043c.

BUG=chromium:636331

Review-Url: https://codereview.chromium.org/2278653003
Cr-Commit-Position: refs/heads/master@{#38916}