[dawn][common] Adds different Notify modes for MutexCondVarProtected.

- Adds an optional template argument for MutexCondVarProtected users
  to explicitly specify notify mode. Sometimes, we may modify the
  inner object but may want to not notify, or notify only one thread.
- Additionally, this change has some cleanups to remove the unused
  MutexProtectedSupport class that I initially added a while back
  thinking it would be useful, but we have since found other ways
  to address the initial use case in MapAsync. It also greatly
  simplifies the code by removing the inheritance that made it quite
  hard to update.

Change-Id: I3c22ecd4ab3fab4149a999ed78474b175c52b1c3
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/290375
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Loko Kung <lokokung@google.com>