In a full mark-compact GC, instead of copying memory to old space for
pages that have more than X% live bytes, we just move the whole page over to old
space.

X=70 (default value)

BUG=chromium:581412
LOG=N

Review URL: https://codereview.chromium.org/1863983002

Cr-Commit-Position: refs/heads/master@{#35610}