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This adds isNavigationEntryMarkedToBeSkipped() to NavigationController
which allows the client to check if an entry will be taken into account
by goBack().

Will look into whether we need some sort of notification for when
entries are marked/unmarked as skipped.

Bug: 1082208
Change-Id: I3b5652e01c76677623d3edb3b97638a4df496fa1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2209591
Commit-Queue: Clark DuVall <cduvall@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#770634}