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This patch replaces forward class declarations |VisiblePosition| to include
directive of "VisiblePosition.h" as preparation of introducing
|VisiblePositionTemplate<Strategy>|, like |PositionAlgorithm<Strategy>|.

This patch is a preparation of making selection to handle granularity for web
component, http://crrev.com/1277863002

BUG=513568
TEST=n/a; No behavior changes

Review URL: https://codereview.chromium.org/1306243006

git-svn-id: svn://svn.chromium.org/blink/trunk@201056 bbb929c8-8fbe-4397-9dbb-9b2b20218538