DM: when 32-bit, disable pathops internal threading by default

DM: when 32-bit, disable pathops internal threading by default

Before this change, when limited to 4G, pathops threaded tests were the weak
link RAM-consumption-wise (in thread-local font caches) up until about 12
cores, where other problems begin to pile up too.

Tested by running:
    ulimit -Sv 4194304
    out/Debug/dm --threads N [--pathOpsSingleThread]

After this, we're _probably_ good to go on 32-bit machines with 8 cores.

BUG=skia:2478
R=borenet@google.com, mtklein@google.com

Author: mtklein@chromium.org

Review URL: https://codereview.chromium.org/265593003

git-svn-id: http://skia.googlecode.com/svn/trunk@14463 2bbb7eff-a529-9590-31e7-b0007b416f81