Remove SkPictureFlat.h include from SkDrawCommands.h

This cannot land until https://codereview.chromium.org/909353004/ (Prepare SkiaBenchmarkingExtensionTest for upcoming Skia changes) lands in Chromium

Committed: https://skia.googlesource.com/skia/+/5a4c233a3657d12d836de388b41e30405b4ab976

Committed: https://skia.googlesource.com/skia/+/fa3a83d0fe53907e618346d3f1c17a95da1a9d22

Review URL: https://codereview.chromium.org/912403004
6 files changed