Assert SkTDArray::pop() doesn't underflow.

Assert SkTDArray::pop() doesn't underflow.

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/aa90d00f14414db39b3d990d8d5fd594d6eadc6e

[mtklein, 2014-09-10 20:57:47]

The CQ bit was checked by mtklein@google.com

[mtklein, 2014-09-10 20:57:56]

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

[commit-bot: I haz the power, 2014-09-10 20:58:35]

CQ is trying da patch. Follow status at
 https://skia-tree-status.appspot.com/cq/mtklein@chromium.org/563633003/1

[commit-bot: I haz the power, 2014-09-10 20:58:36]

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 2014-09-11 02:58 UTC

[commit-bot: I haz the power, 2014-09-10 20:58:39]

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

[commit-bot: I haz the power, 2014-09-10 20:58:40]

Presubmit check for 563633003-1 failed and returned exit status 1.

Running presubmit commit checks ...

** Presubmit ERRORS **
Since the CL is editing public API, you must have an LGTM from one of:
('reed@chromium.org', 'reed@google.com', 'bsalomon@chromium.org',
'bsalomon@google.com', 'djsollen@chromium.org', 'djsollen@google.com')

Presubmit checks took 1.7s to calculate.

[reed1, 2014-09-11 13:26:27]

The CQ bit was checked by reed@google.com

[reed1, 2014-09-11 13:26:27]

lgtm

[commit-bot: I haz the power, 2014-09-11 13:27:30]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patchset/563633003/1

[commit-bot: I haz the power, 2014-09-11 13:36:17]

Committed patchset #1 (id:1) as aa90d00f14414db39b3d990d8d5fd594d6eadc6e