Sync: Handle duplicating tab IDs in foreign session data

Sync: Handle duplicating tab IDs in foreign session data.

This change prevents overwriting a foreign session SessionTab  with a data from another node if the SessionTab we already have has a newer timestamp. This ensures that if there are multiple tabs with the same ID, the tab that we end up associating with a window is the most recent one.

BUG=423501
Committed: https://crrev.com/dab87227950f67f4bd03276cbe84fa914ae846a3
Cr-Commit-Position: refs/heads/master@{#302674}

[stanisc, 2014-11-04 00:16:30]

stanisc@chromium.org changed reviewers:
+ zea@chromium.org

[stanisc, 2014-11-04 00:16:31]

Please review this fix.

[Nicolas Zea, 2014-11-04 18:56:13]

LGTM, nice job!

[stanisc, 2014-11-04 20:40:41]

The CQ bit was checked by stanisc@chromium.org

[commit-bot: I haz the power, 2014-11-04 20:41:11]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/699033002/1

[commit-bot: I haz the power, 2014-11-04 21:27:09]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-11-04 21:27:53]

Patchset 1 (id:??) landed as
https://crrev.com/dab87227950f67f4bd03276cbe84fa914ae846a3
Cr-Commit-Position: refs/heads/master@{#302674}