Make SpaceSplitStringData private and rename to Data

Make SpaceSplitStringData private in the SpaceSplitString class as this is an
implementation detail and there is no need to expose this class.

This CL also renames SpaceSplitStringData to Data to avoid redundancy and fixes
indentation in the header.

R=adamk@chromium.org, rob.buis@samsung.com

Review URL: https://codereview.chromium.org/488593003

git-svn-id: svn://svn.chromium.org/blink/trunk@180652 bbb929c8-8fbe-4397-9dbb-9b2b20218538