Introduce UnicodeUtilities.{h,cpp} and add UnicodeUtilitiesTest.cpp

This patch introduces a set of Unicode utility functions extracted from TextIterator.cpp for using in several places.

BUG=

Review URL: https://codereview.chromium.org/61893022

git-svn-id: svn://svn.chromium.org/blink/trunk@163744 bbb929c8-8fbe-4397-9dbb-9b2b20218538