[TG Sync] Update() should not invoke Connect() on desktop

We currently have UpdateTabGroup() invoke ConnectTabGroup() and
ConnectTabGroup() invoke UpdateTabGroup() if the tab group listeners
aren't initialized yet for sync (typically invoked on startup and
conversion to shared tab group).

With this CL, we remove Connect call from Update. So, the caller when
invoking UpdateTabGroup() should ensure that they are connected already.
The ConnectTabGroup() still continues to invoke UpdateTabGroup(),
because it's harmless to do that, and the caller is spared from invoking
another Update preceded by Connect.

Bug: None
Change-Id: I3b03df020fa7b656b8fb695b13be317aff0a4ff2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5991953
Reviewed-by: Rushan Suleymanov <rushans@google.com>
Commit-Queue: Shakti Sahu <shaktisahu@chromium.org>
Reviewed-by: Darryl James <dljames@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1378499}