Support more than 64 values from NotRestoredReason

Similar to https://crrev.com/c/4584658, this CL makes
NotRestoredReason support 64 or more values with the following
changes:
1. Move the `ToEnumBitmasks()` method from
web_scheduler_tracked_feature to base/enum_set so it can be reused.
2. Use `base::EnumSet` instead of `std::bitset` for
NotRestoredReasons.
3. Update the corresponding usage, and add another UKM entry.

UKM metrics review doc: https://docs.google.com/document/d/1GDsehNwMEHXef_o2XqNbsXk2F0pdhXt1khVVE4cRcLw/edit?usp=sharing


Bug: 333326043
Change-Id: Id10511bbd9f231ddef87869fa2a60e66b0261310
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5432099
Commit-Queue: Mingyu Lei <leimy@chromium.org>
Reviewed-by: Rakina Zata Amni <rakina@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Sun Yueru <yrsun@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1310387}