Fold away SVGInlineTextBoxPainter::paintDecorationWithStyle

The geometry is the same in both the stroke and the fill case, which means
it can be hoisted into the caller (paintDecoration). That leaves only some
fairly trivial code in paintDecorationWithStyle, so fold it up entirely.

BUG=420022

Review URL: https://codereview.chromium.org/654873002

git-svn-id: svn://svn.chromium.org/blink/trunk@183705 bbb929c8-8fbe-4397-9dbb-9b2b20218538