[tracing] Add a DCHECK to ensure that trace events have a valid timestamp.

[tracing] Add a DCHECK to ensure that trace events have a valid timestamp.

Non metadata trace events should not have zero timestamps, so add a DCHECK
that this is the case.

BUG=456940, 456940
Committed: https://crrev.com/1731b978931677a486acae529b0e88d21ad3784f
Cr-Commit-Position: refs/heads/master@{#316226}

[nduca, 2015-02-12 21:27:33]

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

[nduca, 2015-02-12 21:27:34]

lgtm

[rmcilroy, 2015-02-12 21:28:50]

Nat: could you please take a look.  Please see the discussion on
crbug.com/456940 for the reasoning. Thanks.

[rmcilroy, 2015-02-12 21:30:07]

The CQ bit was checked by rmcilroy@chromium.org

[commit-bot: I haz the power, 2015-02-12 21:31:10]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/920963002/1

[commit-bot: I haz the power, 2015-02-12 23:16:46]

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

[commit-bot: I haz the power, 2015-02-12 23:16:47]

Try jobs failed on following builders:
  linux_chromium_rel_ng on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[rmcilroy, 2015-02-13 15:34:06]

The CQ bit was checked by rmcilroy@chromium.org

[commit-bot: I haz the power, 2015-02-13 15:35:03]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/920963002/1

[commit-bot: I haz the power, 2015-02-13 16:32:44]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-02-13 16:33:50]

Patchset 1 (id:??) landed as
https://crrev.com/1731b978931677a486acae529b0e88d21ad3784f
Cr-Commit-Position: refs/heads/master@{#316226}