Add -ftrapv to debug builds.

Add -ftrapv to debug builds.

This adds instrumentation to check that signed integers never under- or
overflow, which is undefined behavior.

BUG=skia:3096
Committed: https://skia.googlesource.com/skia/+/ebacb6127a952910f43a59460af43427d93df46c

[mtklein, 2014-11-05 15:28:59]

mtklein@google.com changed reviewers:
+ bungeman@google.com, mtklein@google.com

[mtklein, 2014-11-05 15:29:02]

The CQ bit was checked by mtklein@google.com

[commit-bot: I haz the power, 2014-11-05 15:29:45]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/702893003/1

[commit-bot: I haz the power, 2014-11-05 15:29:47]

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 valid reviewer(s) till 2014-11-05 21:29 UTC

[commit-bot: I haz the power, 2014-11-05 15:42:16]

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

[commit-bot: I haz the power, 2014-11-05 15:42:17]

Try jobs failed on following builders:
  Test-Ubuntu13.10-GCE-NoGPU-x86_64-Debug-Trybot on client.skia
(http://build.chromium.org/p/client.skia/builders/Test-Ubuntu13.10-GCE-NoGPU-x...)

[mtklein, 2014-11-21 22:09:32]

The CQ bit was checked by mtklein@google.com

[mtklein, 2014-11-21 22:09:33]

lgtm

[commit-bot: I haz the power, 2014-11-21 22:10:21]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/702893003/1

[commit-bot: I haz the power, 2014-11-21 22:24:08]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/ebacb6127a952910f43a59460af43427d93df46c

[mtklein, 2014-11-21 22:42:00]

A revert of this CL (patchset #1 id:1) has been created in
https://codereview.chromium.org/751713002/ by mtklein@google.com.

The reason for reverting is: Not clean yet..