[Document mode] Don't ignore tabs that don't have an initial URL

[Document mode] Don't ignore tabs that don't have an initial URL

Tasks without initial URLs weren't being counted as legitimate Tabs,
preventing them from being restored properly.

BUG=481109
Committed: https://crrev.com/73dfc1c5a87b062a1432713dedb64db2c45dca7d
Cr-Commit-Position: refs/heads/master@{#326944}

[gone, 2015-04-25 00:05:44]

dfalcantara@chromium.org changed reviewers:
+ mariakhomenko@chromium.org

[Maria, 2015-04-25 00:12:06]

lgtm

[gone, 2015-04-25 00:12:21]

The CQ bit was checked by dfalcantara@chromium.org

[commit-bot: I haz the power, 2015-04-25 00:13:01]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1109543003/1

[commit-bot: I haz the power, 2015-04-25 00:36:18]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-25 00:38:10]

Patchset 1 (id:??) landed as
https://crrev.com/73dfc1c5a87b062a1432713dedb64db2c45dca7d
Cr-Commit-Position: refs/heads/master@{#326944}