Improve readability of DesktopDragDropClientOzone code

- Make class variables naming more consistent and readable;
  - Use plural for operation bitmask vars/params
  - Avoid latest/current mixed names which makes it confusing sometimes;
- Add some docs comments;
- Fix some style issues;
- Pure refactor change. No behavior changes.

This precedes the CL which adds WmDropHandler callbacks override order
enforcements as requested in [1].

[1] https://crrev.com/c/5187180/5..10/ui/platform_window/wm/wm_drop_handler.h#31

R=sky

Bug: b/324302980
Change-Id: Id132053bba572aa3229ebbfc13df4d8278789298
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5278425
Commit-Queue: Nick Yamane <nickdiego@igalia.com>
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Auto-Submit: Nick Yamane <nickdiego@igalia.com>
Cr-Commit-Position: refs/heads/main@{#1258268}