Because we now need to maintain state outside of the normal SessionWindow
struct, this patch introduces a SyncedSessionWindow type. It wraps a
normal SessionWindow, while allowing tab sync to track additional window
information.

As part of this patch, I've also cleaned up the unused variation ids.
Moving forward, nothing sync specific should need to be in
SessionWindow. We'll probably need to add a wrapper
for SessionTab in the future as well.

The patch itself does not have any behavioral impact, but paves the way for
consuming sync window type information.

BUG=639009
TBR=bauerb, skuhne, thestig

Review-Url: https://codereview.chromium.org/2499023004
Cr-Commit-Position: refs/heads/master@{#460510}