Check for validity when accessing call site objects in runtime.

Check for validity when accessing call site objects in runtime.

R=jkummerow@chromium.org
BUG=chromium:528379
LOG=N

Committed: https://crrev.com/82b30828bd7d214a2df24125d7008ed97203e52f
Cr-Commit-Position: refs/heads/master@{#31233}

[Yang, 2015-10-13 09:45:22]

The CQ bit was checked by yangguo@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-10-13 09:45:30]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1404613002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1404613002/1

[commit-bot: I haz the power, 2015-10-13 10:25:11]

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

[commit-bot: I haz the power, 2015-10-13 10:25:12]

Dry run: This issue passed the CQ dry run.

[Jakob Kummerow, 2015-10-13 10:48:24]

lgtm

[Yang, 2015-10-13 10:51:36]

The CQ bit was checked by yangguo@chromium.org

[commit-bot: I haz the power, 2015-10-13 10:51:48]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1404613002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1404613002/1

[commit-bot: I haz the power, 2015-10-13 10:53:12]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-10-13 10:53:34]

Patchset 1 (id:??) landed as
https://crrev.com/82b30828bd7d214a2df24125d7008ed97203e52f
Cr-Commit-Position: refs/heads/master@{#31233}