Fix bug where step line isn't drawn immediately.
Fix bug where step line isn't drawn immediately.

Fix it by drawing it on the overlay canvas which is where it probably
belonged in the first place.

BUG=skia:
TBR=mtklein

Review URL: https://codereview.chromium.org/836213003

Tiger66639/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot