Simplify PaintControllerCycleScope

Now that CompositeAfterPaint has launched, there is only one controller
per cycle scope, so the code to manage multiple controllers can be
removed. In addition, the vector of clients to validate can be moved
to be managed by the PaintController itself, rather than externally in
the cycle scope.

Change-Id: Id222c5ad1af88115b12d57d24b1d54a2c08a2f32
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4126172
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Commit-Queue: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1087611}