Cleanup: Make InlineBox have a single public section.

Move private section near where it should be.

This is a continuation of work started here:
https://src.chromium.org/viewvc/blink?view=rev&revision=160660

BUG=None
R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/96683005

git-svn-id: svn://svn.chromium.org/blink/trunk@162927 bbb929c8-8fbe-4397-9dbb-9b2b20218538