Rewrite SVG text-decoration handling to not require GC scale-adjustment

Since drawing decorations for text means drawing "regular geometry" rather
than specially-scaled glyphs of a font, there's no need to first append
the inverse scale-factor the CTM and then scale the geometry - just adjust
the computed (scaled) font-metrics and emit unscaled geometry.
This allows for the stroke thickness adjustment to only be applied in the
text-case, so sink it out of PaintingResourceScope.
Also modify the function that computes the offset for the decorations to
do so relative to the baseline, and rename it accordingly.

Review URL: https://codereview.chromium.org/601763002

git-svn-id: svn://svn.chromium.org/blink/trunk@182615 bbb929c8-8fbe-4397-9dbb-9b2b20218538