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This is a band-aid fix for crbug.com/415167, which I'd like to try to get into M38.
Apparently it's possible to have "temporary" entry IDs set as pending and then
thrown away. This confuses OverscrollNavigationOverlay into thinking that the
page was never loaded and the overlay never gets dismissed as a result, which
makes the webpage unusable.
Using URLs is not as reliable avoids this problem. It can introduce false
positives in some corner cases, however a false positive doesn't have severe
consequences - we simply dismiss the overlay too soon, and the use may see an
empty page or a flicker.

BUG=415167

Review URL: https://codereview.chromium.org/575203002

Cr-Commit-Position: refs/heads/master@{#295811}