Move SegmentedString to Source/platform/text/

BUG=297477
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/25658005

git-svn-id: svn://svn.chromium.org/blink/trunk@158975 bbb929c8-8fbe-4397-9dbb-9b2b20218538