vsplit: Display rotation and transition for top/bottom-snapped window

- Use snap_ratio and `GetSnappedWindowBounds()` to update snapped
window bounds in `AdjustSnappedBounds()` during tablet <> clamshell mode
transition and screen rotation. Thus, rotating display in clamshell mode
can now switch between vertically-snapped and horizontally-snapped
window layout.
- Rename |snapped_width_ratio_| to |snap_ratio_|.
- `UpdateSnappedWidthRatio()` -> `UpdateSnapRatio()`.
- `GetCurrentSnappedWidthRatio()` -> `GetCurrentSnapRatio()`.

Bug: 1236152
Change-Id: Iba20ff2ce1f3e185dc5ab76c35938c4d8080b470
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3054548


Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Commit-Queue: Cattalyya Nuengsigkapian <cattalyya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#908294}