Rework SVGStringList to match SVGProperty object lists

This moves DOM interface operations to the tear-off class, similarly to
how SVGProperty object lists are structured.

Change SVGStringListTearOff to inherit from SVGPropertyTearOffBase
rather than SVGPropertyTearOff<T> since since this type is never
animated, and thus does not need the ability to re-target to another
object.

Bug: 1106762
Change-Id: Ic8040339e3a3cdf6a818333dd77bdf15af3829ce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2302755
Reviewed-by: Stephen Chenney <schenney@chromium.org>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Cr-Commit-Position: refs/heads/master@{#800946}