Remove HistoryItem's copying functionality.

It doesn't appear we need it anymore. It's used in two places: (1) Copying in
preparation for export to the chromium's content/ layer, which mostly exists
to ensure we don't get confused about child HistoryItems, and
(2) WebHistoryItem::ensureMutable(), which exists to make WebHistoyItem copy-
on-write. (1) can easily be worked around by ensuring children are cleared
before exporting the HistoryItem, and (2) appears to be vestigial.

BUG=none

Review URL: https://codereview.chromium.org/214603007

git-svn-id: svn://svn.chromium.org/blink/trunk@170784 bbb929c8-8fbe-4397-9dbb-9b2b20218538