Move skp nanobench tile size back to 256x256 to see if it fixes nanobench crashes

Going back to old nanobench tile size to see if the increase to tile is what has been
causing recent nanobench crashes. The crashes seem very nondeterministic and hard to
debug manually.

256x256 is too small of a tile to give accurate gpu results but if this fixes we can try some compromise in the middle

BUG=skia:

Review URL: https://codereview.chromium.org/1022823003
1 file changed