SkCanvas::resetForNextPicture()

No diffs against head for DM --config 8888 gpu 2ndpic-8888 2ndpic-gpu.

  picture_overhead_draw	1.62us ->  1.6us	0.99x
picture_overhead_nodraw	 792ns ->  342ns	0.43x

tiles and serialization modes will also test this a bit.

BUG=chromium:470553

Review URL: https://codereview.chromium.org/1067893002
7 files changed