AI 144443: Change how skia handles gradient stops that are missing either pos=0 or...

AI 144443: Change how skia handles gradient stops that are missing either pos=0 or pos=1. Instead of ignoring the first and last pos, and assuming they were 0,1, now we insert in the missing values. This matches webkit's expectations, and seems fine for skia clients as well.
  On the webkit side, call Gradient::getColor() before we sniff the stops-array. This has the side-effect of sorting the stops, which we need for skia.
  BUG=1688373

Automated import of CL 144443