Compaction selection now derives target fragmentation based on compaction speed
and a goal per page, if compaction speed has been traced. This way we avoid
overlong compaction of single pages.

This change could regress memory consumption if (a) the memory reducer does not
kick in, and (b) lots of medium fragmented pages would be compacted in our fast
path.

BUG=chromium:524425
LOG=N

Review URL: https://codereview.chromium.org/1480553004

Cr-Commit-Position: refs/heads/master@{#32361}