Reason for revert:
Fragmentation of LABs could result in increasing memory usage (pages) instead of shrinking.
BUG=chromium:620320
LOG=N
Original issue's description:
> [heap] Add page evacuation mode for new->new
>
> Adds an evacuation mode that allows moving pages within new space without
> copying objects.
>
> Basic idea:
> a) Move page within new space
> b) Sweep page to make iterable and process ArrayBuffers
> c) Finish sweep till next scavenge
>
> Threshold is currently 70% live bytes, i.e., the same threshold we use
> to determine fragmented pages.
>
> BUG=chromium:581412
> LOG=N
> CQ_EXTRA_TRYBOTS=tryserver.v8:v8_linux_arm64_gc_stress_dbg,v8_linux_gc_stress_dbg,v8_mac_gc_stress_dbg,v8_linux64_tsan_rel,v8_mac64_asan_rel
>
> Committed: https://crrev.com/49b23201671b25092a3c22eb85783f39b95a5f87
> Cr-Commit-Position: refs/heads/master@{#36990}
TBR=ulan@chromium.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=chromium:581412
Review-Url: https://codereview.chromium.org/2063013005
Cr-Commit-Position: refs/heads/master@{#37042}
2263ee9bf4