Only use 256x256 tiles on hd2000 nanobench bots

Initial experiments did show that the 256 tile size fixed the hd2000 win7
nanobot failures. However it did not have any effect on other bots, so this
change is to move back to the larger tile size on all bots expect for the
hd2000.

BUG=skia:

Review URL: https://codereview.chromium.org/1022083002
3 files changed