Use Position instead of VisiblePosition for SurroundingText.

Using VisiblePosition has some not required performance implications.

BUG=None

Review URL: https://codereview.chromium.org/307353002

git-svn-id: svn://svn.chromium.org/blink/trunk@175788 bbb929c8-8fbe-4397-9dbb-9b2b20218538