Some inline ARM assembly for saturated arithmetic and a small speed-up
for the C version of saturated add and subtract. Created a unit test for
setting a saturated value.

Benchmarking using rasterize_and_record_micro.key_mobile_sites on a
Nexus7 recorded a consistent drop in RasterizeAndRecordMicro:record_time
from 5.18ms to 4.93ms (~5% improvement).

BUG=325746

Review URL: https://codereview.chromium.org/346913004

git-svn-id: svn://svn.chromium.org/blink/trunk@177208 bbb929c8-8fbe-4397-9dbb-9b2b20218538