Fix deserialization of unmapped tabs

An unmapped tab can still have an associated |tab_node_id| (e.g. a
foreign tab that has been closed), so the assumption behind the CHECK
prior to this patch was wrong, and the implementation buggy.

According to the reports we've investigated, this scenario occurs for
both local and remote sessions.

Bug: 837517
Change-Id: Ic436fe9eacce131fef5d0394bdfff34d9aedc5b3
Reviewed-on: https://chromium-review.googlesource.com/1044211
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556188}
3 files changed