GitLab

This CL makes the ContentsView's API state-keyed rather than page index
keyed. This removes cruft from callers of SetActivePage and will
support further refactors.

BUG=455059
TBR=jamescook@chromium.org, benwells@chromium.org

Review URL: https://codereview.chromium.org/947903002

Cr-Commit-Position: refs/heads/master@{#321513}