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Converts TreeNodeModel to allow an index-based removal for cases where
an index is known. This prevents a linear search in that case. A
pass-through function is provided that does the search if the index is
not known.

This requires a similar change in TreeNode. TreeNode::Add and ::Remove
used to be virtual but these were never overridden. They are changed
to non-virtual member functions.

Additional documentation is added referencing TreeNodeModel from TreeModel, and
some additional examples are provided.

Change-Id: I5c062420bada437f1ef127ce80518e37f5c2aaae
Reviewed-on: https://chromium-review.googlesource.com/820671
Commit-Queue: Brett Wilson <brettw@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523591}