Fix used vs. capacity growth computation mismatch.

Fix used vs. capacity growth computation mismatch.

Free space was not being taking into consideration,
effectively growing more when more garbage was collected.

This change makes the growth uniform. (In the future,
we probably want to grow more when less was collected.)

R=iposva@google.com

Committed: https://code.google.com/p/dart/source/detail?r=34909

[Ivan Posva, 2014-04-10 00:15:12]

LGTM -ip

[koda, 2014-04-10 00:15:27]

As expected, this change makes SplayHarder slower (up to 20%):

http://108.170.219.2:8080/Comparison#targetA%3Ddart%3BmachineTypeA%3Dlinux-ia...

This can be counteracted by increasing heap_growth_space_ratio to 20% from
today's 10%.

[koda, 2014-04-10 00:24:25]

Committed patchset #3 manually as r34909 (presubmit successful).