Improve DCHECK in PropertyTree::UpdateSnapping

Improve DCHECK in PropertyTree::UpdateSnapping

We DCHECK that the snap matrix is indentity or a 2D translation, but the
computation of this matrix is prone to floating point error, so an
approximate check makes more sense here.

BUG=None

Committed: https://crrev.com/e44d0bb8e278e241157df15962d138f03521d512
Cr-Commit-Position: refs/heads/master@{#324247}

[ajuma, 2015-04-08 14:38:01]

ajuma@chromium.org changed reviewers:
+ ajuma@chromium.org

[ajuma, 2015-04-08 14:38:02]

lgtm

[Ian Vollick, 2015-04-08 14:40:31]

The CQ bit was checked by vollick@chromium.org

[commit-bot: I haz the power, 2015-04-08 14:40:36]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1067423002/1

[commit-bot: I haz the power, 2015-04-08 15:44:00]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-04-08 15:44:01]

Try jobs failed on following builders:
  win_chromium_compile_dbg_ng on tryserver.chromium.win (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.win/builders/win_chromium_comp...)

[Ian Vollick, 2015-04-08 15:51:30]

The CQ bit was checked by vollick@chromium.org

[Ian Vollick, 2015-04-08 15:51:31]

The patchset sent to the CQ was uploaded after l-g-t-m from ajuma@chromium.org
Link to the patchset: https://codereview.chromium.org/1067423002/#ps20001
(title: "SkDoubleToMScalar")

[commit-bot: I haz the power, 2015-04-08 15:52:12]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1067423002/20001

[commit-bot: I haz the power, 2015-04-08 17:26:08]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-04-08 17:26:55]

Patchset 2 (id:??) landed as
https://crrev.com/e44d0bb8e278e241157df15962d138f03521d512
Cr-Commit-Position: refs/heads/master@{#324247}