SVGAnimatedNumber initial values

This implements initial values for SVGAnimatedNumber properties (which
includes SVGAnimatedNumberOptionalNumber since it is a reverse proxy to
the former.)

Bug: 225807
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Icb62d7383cacf7df122fc56af0977015b5adf9b7
Reviewed-on: https://chromium-review.googlesource.com/1209705
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589219}
21 files changed