Change tilegrid test to test it directly, rather than through SkPicture

Change tilegrid test to test it directly, rather than through SkPicture
This is necessary because it makes assumptions that picture will draw all the rects that match the grids, which may not hold if picture decides to improve the accuracy of the results.

BUG=skia:2125
R=tomhudson@google.com, mtklein@google.com, reed@google.com

Author: iancottrell@google.com

Review URL: https://codereview.chromium.org/199083004

git-svn-id: http://skia.googlecode.com/svn/trunk@13827 2bbb7eff-a529-9590-31e7-b0007b416f81