Consider the start_coord in interlaced PNG

crrev.com/2420843003 (DIFFERENT ISSUE) resulted in a slight difference
in Gold for interlaced PNGs. The new images appeared to have slid down
slightly, implying that we sampled pixels higher (earlier) in the
image.

It turns out we were not truly taking get_start_coord into account.
This CL initializes the srcRow to consider get_start_coord, and should
fix the problem/difference.
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2424353003

Review-Url: https://codereview.chromium.org/2424353003
1 file changed