Add window parameter to GameMode::Observer

This change adds a window parameter to the GameMode::Observer callback
method. This allows observers to be aware of which window the game mode
currently refers to.

Bug: 309969845
Change-Id: Ia2d4a184814a3792d62244f7f9c20857847eed2d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5259909
Reviewed-by: Drew Davenport <ddavenport@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Andrew Wolfers <aswolfers@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1264660}