Remove FinalizePageSequentially as it had only a single use case that
was tied to the full collector.

Bug: chromium:651354
Change-Id: I03299ddbd439ea273e02dd33f12c005371694130
Reviewed-on: https://chromium-review.googlesource.com/504508Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45280}