This doesn't add them to the second-stringer Sk4i. It's unclear we should be
doing that often, and we don't have efficient ways to do it except via floats.
BUG=skia:
Review URL: https://codereview.chromium.org/964603002
work on tests
CQ_EXTRA_TRYBOTS=client.skia:Test-Ubuntu13.10-GCE-NoGPU-x86_64-Debug-ASAN-Trybot,Test-Ubuntu12-ShuttleA-GTX660-x86-Debug-Trybot,Test-Win7-ShuttleA-HD2000-x86_64-Debug-Trybot,Test-Win7-ShuttleA-HD2000-x86-Debug-Trybot
BUG=skia:
Review URL: https://codereview.chromium.org/704923003