Swap iteration order in TileGrid::insert().

Swap iteration order in TileGrid::insert().

Was looking at performance here (it's the record hotspot) and noticed we
iterate through the grid out of order.  This is a tiny little thing, but it's
probably orthogonal to any other performance improvements we'll make in here.

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/eee606c7bf6b343429dcdbf56b6b71bd83575060

[mtklein, 2014-09-24 14:25:04]

The CQ bit was checked by mtklein@google.com

[mtklein, 2014-09-24 14:25:12]

mtklein@google.com changed reviewers:
+ robertphillips@google.com

[commit-bot: I haz the power, 2014-09-24 14:25:35]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/598933004/1

[commit-bot: I haz the power, 2014-09-24 14:25:36]

Note for Reviewers:
The CQ is waiting for an approval.
If you believe that the CL is not ready yet, or if you would like to L-G-T-M
with comments then please uncheck the CQ checkbox.

Waiting for LGTM from a valid reviewer till 2014-09-24 20:25 UTC

[robertphillips, 2014-09-24 14:35:50]

lgtm

[mtklein, 2014-09-24 15:55:31]

The CQ bit was unchecked by mtklein@google.com

[mtklein, 2014-09-24 15:55:33]

The CQ bit was checked by mtklein@google.com

[commit-bot: I haz the power, 2014-09-24 15:55:36]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/598933004/1

[commit-bot: I haz the power, 2014-09-24 15:59:42]

Committed patchset #1 (id:1) as eee606c7bf6b343429dcdbf56b6b71bd83575060