We use a scratchpad to remember visited allocation sites for post processing
(making tenure decisions). The previous implementation used a rooted FixedArray
with constant length (256) to remember all sites. Updating the scratchpad is a
bottleneck in any parallel/concurrent implementation of newspace evacuation.

The new implementation uses a HashMap with allocation sites as keys and
temporary counts as values. During evacuation we collect a local hashmap of
visited allocation sites. Upon merging the local hashmap back into a global one
we update potential forward pointers of compacted allocation sites.  The
scavenger can directly enter its entries into the global hashmap. Note that the
actual memento found count is still kept on the AllocationSite as it needs to
survive scavenges and full GCs.

BUG=chromium:524425
LOG=N
R=hpayer@chromium.org

Review URL: https://codereview.chromium.org/1535723002

Cr-Commit-Position: refs/heads/master@{#33233}