Convert the rest of enums in FrameSelection.h into enum classes.
Pull request: https://github.com/WebKit/WebKit/pull/14112
Committed 264288@main (05b6a82d7213): <https://commits.webkit.org/264288@main> Reviewed commits have been landed. Closing PR #14112 and removing active labels.
<rdar://problem/109599190>