Rework SVG*TearOff types back-referencing

In preparation for making base-val <-> attribute synchronization,
refactor the SVG*TearOff hierarchy so that "mutation notifications" are
funneled through the SVGAnimatedProperty(Base) if the tear-off is
associated with an attribute.

Rather than storing a SVGElement* and QualifiedName, store a reference
to the underlying SVGAnimatedProperty(Base) instead of the latter. This
is then used to send notification through.

BUG=873470

Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Ib6e5c25cf97b806ff613a39478dc4f10f85dcaf5
Reviewed-on: https://chromium-review.googlesource.com/1181342
Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Cr-Commit-Position: refs/heads/master@{#584888}
31 files changed