Failing to do the right check in AdvancePage results in a crash in a CHECK later
in EnsureCurrentCapacity.

BUG=chromium:620750,chromium:622115
LOG=N
R=jochen@chromium.org

Review-Url: https://codereview.chromium.org/2090013002
Cr-Commit-Position: refs/heads/master@{#37171}