New algorithm for selecting evacuation candidates

New algorithm for selecting evacuation candidates

This lifts the sqrt(n) limit on number of evacuation candidates,
replaces O(n * sqrt(n)) algorithm with O(n*log(n)) algorithm, and
removes hard-coded constants.

Evacuation candidates are selected as follows:

1) Sort pages from the most free to the least free.

2) Select the first m pages as evacuation candidates such that m is as
large as possible under the two conditions:

- The total size of live objects in the first m pages does not exceed
the given limit. This is based on the assumption that the evacuation cost is
proportional to the total size of moved objects.

- The fragmentation of the (m+1)-th page does not exceed the given
limit.

Review URL: https://codereview.chromium.org/1038313003

Cr-Commit-Position: refs/heads/master@{#28651}