Add Clone and disallow copy construction for NavigationEntryImpl.

This will matter as NavigationEntries start to share FrameNavigationEntries
for --site-per-process.  At that point, cloning a NavigationEntry will need
to take into account whether the copy will live in the same tab or not, as
well as making a deep copy of the FrameNavigationEntry tree.

BUG=236848
TEST=No behavior change yet.

Review URL: https://codereview.chromium.org/994803004

Cr-Commit-Position: refs/heads/master@{#320338}
6 files changed