Don't dispatch two 'load' events for external SVGScriptElement loads

If inserting an <svg:script> reference an external source via script,
ScriptLoaded would dispatch 'load' and then SVGScriptElement would
do the same. Drop the latter one.
Since this makes SVGScriptElement no longer use
SVGElement::sendSVGLoadEventIfPossibleAsynchronously(), drop the
svgLoadEventTimer() override and associated timer.

Review URL: https://codereview.chromium.org/1141683003

git-svn-id: svn://svn.chromium.org/blink/trunk@195604 bbb929c8-8fbe-4397-9dbb-9b2b20218538
4 files changed