Portals: Declare DocumentPortals as GC mixin.

This fixes a GC crash.

Change-Id: Ie5fcfdee033e8f2b45282bfc88d1210bf3a8d861
Reviewed-on: https://chromium-review.googlesource.com/c/1338335Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Commit-Queue: Lucas Gadani <lfg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608510}

third_party/blink/renderer/core/html/portal/document_portals.h

@@ -15,6 +15,8 @@ class HTMLPortalElement;
 
 class DocumentPortals : public GarbageCollected<DocumentPortals>,
                         public Supplement<Document> {
+  USING_GARBAGE_COLLECTED_MIXIN(DocumentPortals);
+
  public:
   static const char kSupplementName[];
   static DocumentPortals& From(Document&);