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.

BUG=chromium:636331

This reverts commit 555c9619.

Review-Url: https://codereview.chromium.org/2232653003
Cr-Commit-Position: refs/heads/master@{#38608}