Make Position::isRenderedCharacter() to work only with null-position and OffsetInAnchor position
Make Position::isRenderedCharacter() to work only with null-position and OffsetInAnchor position

This patch changes |PositionAlgorithm<Strategy>::isRenderedCharacter()| to work
with null-position and |OffsetInAnchor| position anchor type to make usage of
offset in |isRenderedCharacter()| to clear for improving readability.

This patch is a preparation of getting rid of legacy editing position.

BUG=513114
TEST=n/a; no behavior changes

Review URL: https://codereview.chromium.org/1267233002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@200007 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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