Mike's radial gradient CL with better float -> int.

Mike's radial gradient CL with better float -> int.

patch from issue 1072303005 at patchset 40001 (http://crrev.com/1072303005#ps40001)

This looks quite launchable.  radial_gradient3, min of 100 samples:
  N5:  985µs -> 946µs
  MBP: 395µs -> 279µs

On my MBP, most of the meat looks like it's now in reading the cache and writing to dst one color at a time.  Is that something we could do in float math rather than with a lookup table?

BUG=skia:

CQ_EXTRA_TRYBOTS=client.skia.compile:Build-Mac10.8-Clang-Arm7-Debug-Android-Trybot,Build-Ubuntu-GCC-Arm7-Release-Android_NoNeon-Trybot

Committed: https://skia.googlesource.com/skia/+/abf6c5cf95e921fae59efb487480e5b5081cf0ec

Review URL: https://codereview.chromium.org/1109643002