RFR: 8189405: More cleanup in HtmlWriter

Mon Oct 16 22:37:33 UTC 2017

Please review a medium-simple fix to continue the HtmlWriter cleanup 
started in JDK-8160697.
This is about removing "constants" from the per-page HtmlWriter, such 
that they are either moved
to the shared Contents class, or just created as needed.

This is done in part by introducing a new TableHeader class to help 
create headers for tables.

This is cleanup only: no new tests or change in generated docs.

JBS: https://bugs.openjdk.java.net/browse/JDK-8189405
Webrev: http://cr.openjdk.java.net/~jjg/8189405/webrev.00/

-- Jon