Add microbench for shadows

Based on Android's shadowgrid2.

Change-Id: I33209b6b84fc63a762405c4988afbdbffcc170d6
Reviewed-on: https://skia-review.googlesource.com/17360
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
2 files changed