Oilpan: disable conservative GCs during initial GC mixin construction.

Source/core/svg/SVGAElement.h

Index: Source/core/svg/SVGAElement.h
diff --git a/Source/core/svg/SVGAElement.h b/Source/core/svg/SVGAElement.h
index a2c83aca5e3b9c023f2763265f09b10db8c2983b..a63b13086b078ca971214d1019129467442859e0 100644
--- a/Source/core/svg/SVGAElement.h
+++ b/Source/core/svg/SVGAElement.h
@@ -31,7 +31,7 @@ namespace blink {
 class SVGAElement final : public SVGGraphicsElement,
                           public SVGURIReference {
     DEFINE_WRAPPERTYPEINFO();
-    WILL_BE_USING_GARBAGE_COLLECTED_MIXIN(SVGAElement);
+    WILL_BE_USING_GARBAGE_COLLECTED_MIXIN_NESTED(SVGAElement, SVGGraphicsElement);
 public:
     DECLARE_NODE_FACTORY(SVGAElement);
     SVGAnimatedString* svgTarget() { return m_svgTarget.get(); }