Replace SVG*List::clone() by a helper template

Currently there are too much common code in SVG*List.cpp.
This CL replaces SVG*List::clone() which has almost identical implementations
with SVGListPropertyHelper::clone().

Note that SVG{Length,PathSeg}List::clone() are left as is, as they need special handling:
- SVGPathSeg itself is a tearoff.
- SVGLengthList needs to keep |m_mode|.

BUG=378670

Review URL: https://codereview.chromium.org/338303002

git-svn-id: svn://svn.chromium.org/blink/trunk@176644 bbb929c8-8fbe-4397-9dbb-9b2b20218538