Move selectedText() to Frame/FrameSelection from Editor

This patch moves implementation of selectedText() to FrameSelection and exposing via Frame instead of Editor for reducing size of Editor class and reduce dependency of Editor class. Because of Frame class owns selection both logically and implementation, it is intuitive to retrieve selected text via Frame rather than Editor.

BUG=n/a
TEST=n/a; no behavior changes
R=tkent@chromium.org

Review URL: https://codereview.chromium.org/22556003

git-svn-id: svn://svn.chromium.org/blink/trunk@155732 bbb929c8-8fbe-4397-9dbb-9b2b20218538