Treat (private, internal) grid bounds as doubly-inclusive in SkTileGrid.

Treat (private, internal) grid bounds as doubly-inclusive in SkTileGrid.

The net effect is that two "+1" instructions are removed from insert().
search() nets no change: two +1 removed, two +1 added.

When vectorized, this removes the need to add in userToGrid() at all and
so the need to read an awkward {0, 0, 1, 1} constant from memory.  Mostly
the benefit is less vector code to look at and think about.

BUG=skia:

Review URL: https://codereview.chromium.org/659823004