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This is the web/ analogue of content's NavigationEntry, intended
for use on iOS because iOS cannot depend on content.
NavigationItem captures all the data required to recreate a
browsing state, such as URL, title, and transition type.

BUG=422033
TEST=None

Review URL: https://codereview.chromium.org/647543002

Cr-Commit-Position: refs/heads/master@{#299093}