Create an SkCodecImageGenerator

BUG=skia:

patch from issue 1396323007 at patchset 120001 (http://crrev.com/1396323007#ps120001)
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1487683004

Committed: https://skia.googlesource.com/skia/+/e1102ce1d3d0895e840e756e155ec56b5a1a7540

Review URL: https://codereview.chromium.org/1487683004
4 files changed