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Because the pagemap uses the term present instead of in core
we will rename those methods.

Additionally, we add a little helper called IsFullyPresent.

Bug: chromium:1067833
Change-Id: I98099cdd1afa5ed0bc3ded3f22592000719281a1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2462687Reviewed-by: Kuo-Hsin Yang <vovoy@chromium.org>
Commit-Queue: Brian Geffon <bgeffon@chromium.org>
Auto-Submit: Brian Geffon <bgeffon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815441}