Use PartitionAlloc for fastMalloc on Android

This switches fastMalloc to PartitionAlloc for Android. Previously, we only
used PartitionAlloc for fastMalloc on Mac OS X.

According to primiano's measurements, this CL should improve PLT performance
on Android by 7%.  See referenced bug for details.

alexa_us on N10 with default alloc (without
#     ms/page  PSS   Dirty
#1    227.6   60410 44904
#2    226.1   61547 46008
#3    222.4   55595 40072
#4    224.2   57293 41728
AVG   225.1   58711 43178

alexa_us on N10 with partalloc
#     ms/page  PSS   Dirty
#1    204.68  54862  39456
#2    208.73  59240  43844
#3    208.57  55164  39788
#4    217.28  58280  42904
AVG   209.81  56886  41498

BUG=312725
R=tonyg@chromium.org

Review URL: https://codereview.chromium.org/148013010

git-svn-id: svn://svn.chromium.org/blink/trunk@166271 bbb929c8-8fbe-4397-9dbb-9b2b20218538