Add StyleBuilderConverter::convertFlags.

This gets rid of the special style building for text-decoration(-line)
and touch-action properties.

Review URL: https://codereview.chromium.org/424683004

git-svn-id: svn://svn.chromium.org/blink/trunk@179388 bbb929c8-8fbe-4397-9dbb-9b2b20218538