Protect access to the external_snapshot_blob global with a lock.

Protect access to the external_snapshot_blob global with a lock.

The external_snapshot_blob is a global and might be accessed from multiple
threads.  Protect it with a lock.

BUG=457656
LOG=N
R=vogelheim@chromium.org

Committed: https://crrev.com/1da1e34788eca083eec26492e16da76f8520eee4
Cr-Commit-Position: refs/heads/master@{#26638}

[rmcilroy, 2015-02-12 15:52:30]

rmcilroy@chromium.org changed reviewers:
+ vogelheim@chromium.org

[rmcilroy, 2015-02-12 15:52:31]

PTAL, thanks.

[vogelheim, 2015-02-12 17:49:24]

lgtm

[rmcilroy, 2015-02-12 17:55:08]

The CQ bit was checked by rmcilroy@chromium.org

[commit-bot: I haz the power, 2015-02-12 17:56:03]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/918213002/1

[commit-bot: I haz the power, 2015-02-12 18:02:14]

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

[commit-bot: I haz the power, 2015-02-12 18:02:15]

Try jobs failed on following builders:
  v8_linux64_rel on tryserver.v8 (JOB_FAILED,
http://build.chromium.org/p/tryserver.v8/builders/v8_linux64_rel/builds/2951)

[rmcilroy, 2015-02-13 07:58:47]

The CQ bit was checked by rmcilroy@chromium.org

[commit-bot: I haz the power, 2015-02-13 07:59:44]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/918213002/1

[commit-bot: I haz the power, 2015-02-13 08:05:35]

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

[commit-bot: I haz the power, 2015-02-13 08:05:37]

Try jobs failed on following builders:
  v8_linux64_rel on tryserver.v8 (JOB_FAILED,
http://build.chromium.org/p/tryserver.v8/builders/v8_linux64_rel/builds/2973)

[rmcilroy, 2015-02-13 10:36:03]

Committed patchset #1 (id:1) manually as
1da1e34788eca083eec26492e16da76f8520eee4 (presubmit successful).

[commit-bot: I haz the power, 2015-02-13 10:36:06]

Patchset 1 (id:??) landed as
https://crrev.com/1da1e34788eca083eec26492e16da76f8520eee4
Cr-Commit-Position: refs/heads/master@{#26638}