Move non-trivial work in RectBench() to onPreDraw.

Saw this on a profile where rects was filtered out.  ಠ_ಠ

BUG=
R=reed@google.com

Author: mtklein@google.com

Review URL: https://codereview.chromium.org/93703003

git-svn-id: http://skia.googlecode.com/svn/trunk@12500 2bbb7eff-a529-9590-31e7-b0007b416f81
1 file changed