Start to vectorize SkTileGrid.

This adds Sk4x.h to help.

BUG=skia:

Review URL: https://codereview.chromium.org/634543004
Revert of Start to vectorize SkTileGrid. (patchset #45 id:1430002 of https://codereview.chromium.org/634543004/)

Reason for revert:
Many GCC bots missing __builtin_shuffle, e.g. Test-Ubuntu12-ShuttleA-GTX660-x86-Debug-Trybot.

Original issue's description:
> Start to vectorize SkTileGrid.
>
> This adds Sk4x.h to help.
>
> BUG=skia:
>
> Committed: https://skia.googlesource.com/skia/+/90c7992bfc6330f070f7704d63372a0ec8410170

TBR=reed@google.com,mtklein@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=skia:

Review URL: https://codereview.chromium.org/663663002