[SBv5] Convert enum in safe_browsing_api_handler_util.h to enum class

This will help resolve naming conflicts as we introduce very similar
set of enums for SafeBrowsing API. It is also the preferred use of
enum (go/totw/86#conclusion).

Pure refactoring CL, no behavioral change.

Bug: 1444511
Change-Id: I1c68e3f3e9e375989d684144dfd849a13479fbb7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4701695
Reviewed-by: thefrog <thefrog@chromium.org>
Commit-Queue: Xinghui Lu <xinghuilu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1177902}