Cleanup ash::WallpaperController.

- Add ResetProminentColors().
- Convert some ints to base::TimeDelta.
- Fix comments with the wrong style.
- Use early returns in a few places.
- Simplify code in a bunch of places.

Change-Id: Iab562eb3883f930b83c9b85fcc7cdb4fb0f6f806
Reviewed-on: https://chromium-review.googlesource.com/1063317
Reviewed-by: Wenzhao (Colin) Zang <wzang@chromium.org>
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#559644}