Fix DCHECK caused by specifying a negative timestamp offset.

Fix DCHECK caused by specifying a negative timestamp offset.

Timestamp offsets are allowed to be negative, but a DCHECK in
DoubleToDelta() was causing a crash in debug builds. This patch simply
moves the "not negative" DCHECK to the methods that actually need to
verify that constraint.

BUG=353245
TEST=LayoutTest that verifies this fix is in https://codereview.chromium.org/202423002/.

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=257657

[wolenetz, 2014-03-17 20:11:49]

lgtm

[acolwell GONE FROM CHROMIUM, 2014-03-17 20:18:53]

The CQ bit was checked by acolwell@chromium.org

[commit-bot: I haz the power, 2014-03-17 20:19:52]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/acolwell@chromium.org/198213008/1

[commit-bot: I haz the power, 2014-03-18 05:56:26]

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

[commit-bot: I haz the power, 2014-03-18 05:56:27]

Commit queue rejected this change because the description was changed
between the time the change entered the commit queue and the time it
was ready to commit. You can safely check the commit box again.

[acolwell GONE FROM CHROMIUM, 2014-03-18 16:16:26]

The CQ bit was checked by acolwell@chromium.org

[commit-bot: I haz the power, 2014-03-18 16:17:28]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/acolwell@chromium.org/198213008/1

[commit-bot: I haz the power, 2014-03-18 16:24:04]

Change committed as 257657