[blink scheduler] Minor refactoring in advance of new UseCase

This CL refactors a few things to make the diffs simpler when the new
UseCase and associated QueueTrait are added:

 - UseCase
   - Change kCount to kMaxValue and remove default case in switch
     statements
   - Add a helper for computing the RAILMode from UseCase to make
     UpdatePolicyLocked easier to read
   - Move UseCaseToString into use_case.cc as to be with related code.
   - Reorder the enum to group similar use cases (this is no longer
     associated with any UMAs)
 - MainThreadTaskQueue::QueueTraits
  - Initialize members in declaration and default the c'tor
  - Change operator== to be default (vs. member-wise ==)

Bug: 350540984
Change-Id: Ie66b5080c6bb537a48eeb538e164f331ab19b4e6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5663989
Reviewed-by: Alexander Timin <altimin@chromium.org>
Commit-Queue: Scott Haseley <shaseley@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1323038}