Simplify remapping of obsolete session IDs

The new implementation removes some dead code (instead use DCHECKs) and
avoids unnecessary remappings when no tabbed windows exist (e.g.
multiple custom tabs exist without native data).

Bug: 843554
Change-Id: I595516158cc8e5aafb13346a962bf92f70ea6aad
Reviewed-on: https://chromium-review.googlesource.com/1068674
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561036}
3 files changed