Allow Web Animations CSS flag to be turned off as well as on

This is a first step towards switching to Web Animations as the default
implementation of CSS animations and transitions. When that switch is made, a
virtual test suite will be added to run the animations and transitions
LayoutTests with the legacy animations engine. Those virtual suites will
require this --disable-web-animations-css flag.

Also pass an explicit value to WebRuntimeFeatures::enableWebAnimationsSVG(), to
allow the default value to be removed from Blink.

BUG=258896
R=dstockwell@chromium.org, jam@chromium.org, dstockwell

Review URL: https://codereview.chromium.org/61643025

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235301 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235301