TabIdManager and SessionIdGenerator seem very identical these days after SessionID-generation has recently become more sophisticated and makes session IDs unique across restarts (see crbug.com/823798).
Unifying the two would simplify the codebase in general (I'd say, minor improvement) and, in case crbug.com/851905 gets addressed, be beneficial for sync, because no collisions would exist between session IDs and Android IDs (notable improvement).
Comment 1 by mastiz@chromium.org
, Jun 18 2018