Let Position-based findPlainText return Positions as results.

blink::TextFinder uses findPlainText() function to search text. To be able to
search a page containing Shadow DOM properly, it needs to use Positions,
instead of Range, for the document range to be searched and for ranges of
text found.

This patch makes Position-based findPlainText() return the range of text found
as Positions instead of Range, as a step toward fixing find-in-page with
Shadow DOM.

Refactoring only; behavior should not change.

BUG=353430

Review URL: https://codereview.chromium.org/227153005

git-svn-id: svn://svn.chromium.org/blink/trunk@171012 bbb929c8-8fbe-4397-9dbb-9b2b20218538