[svg] Remove unnecessary write barrier

SVGPropertyTearOffBase::context_element_ is already TraceWrapperMember
and will thus emit a write barrier for wrapper tracing.

Bug: chromium:843903
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I89d540179dc47792172f75bf769b898f0290c59e
Reviewed-on: https://chromium-review.googlesource.com/1190163Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#586250}

third_party/blink/renderer/core/svg/properties/svg_property_tear_off.cc

@@ -66,9 +66,6 @@ void SVGPropertyTearOffBase::Bind(SVGAnimatedPropertyBase* binding) {
   DCHECK(binding);
   DCHECK(binding->ContextElement());
   context_element_ = binding->ContextElement();
-  // Requires SVGPropertyTearOffBase to be the left-most class in the
-  // inheritance hierarchy.
-  ScriptWrappableMarkingVisitor::WriteBarrier(context_element_.Get());
   binding_ = binding;
 }