Copy TextIterator.{cpp.h} to UnicodeUtilities.{cpp,h}

This patch copies Source/core/editing/TextIterator.{cpp,h} into Source/core/platform/text/UnicodeUtilities.{cpp,h} for decoupling TextIterator and a set of Unicode utility functions.

Another patch will complete decoupling.

BUG=

Review URL: https://codereview.chromium.org/69673004

git-svn-id: svn://svn.chromium.org/blink/trunk@162732 bbb929c8-8fbe-4397-9dbb-9b2b20218538