Draw a single underline for text-decoration-style: [dashed, dotted].

There is a bug in the new text-decoration styles where the dashed and dotted
underlines would be drawn twice. This appears to be because we are stroking
a SkRect of height zero.

Fixed by drawing a line rather than stroking a rect.

BUG=348518
TEST=Covered by existing tests

Review URL: https://codereview.chromium.org/185683003

git-svn-id: svn://svn.chromium.org/blink/trunk@168743 bbb929c8-8fbe-4397-9dbb-9b2b20218538