Extend WebSurroundingText to accept a WebRange.

This will make it possible to get the text surrounding the
current selection by calling WebFrame::selectionRange() and
passing that range to WebSurroundingText.

BUG=330238

Review URL: https://codereview.chromium.org/294073005

git-svn-id: svn://svn.chromium.org/blink/trunk@176168 bbb929c8-8fbe-4397-9dbb-9b2b20218538