GitLab

Move serving_dirs to UserStorySet, delete Page.archive_path (instead using PageSet.WprFilePathForUserStory).
Update user_story_runner to no longer gate archive-specific logic with instanceof PageSet check.

BUG=439512

Review URL: https://codereview.chromium.org/725133005

Cr-Commit-Position: refs/heads/master@{#308134}