diff -up chromium-65.0.3325.146/third_party/WebKit/Source/platform/heap/TraceTraits.h.oilpan chromium-65.0.3325.146/third_party/WebKit/Source/platform/heap/TraceTraits.h
--- chromium-65.0.3325.146/third_party/WebKit/Source/platform/heap/TraceTraits.h.oilpan	2018-03-13 16:45:40.613426445 -0400
+++ chromium-65.0.3325.146/third_party/WebKit/Source/platform/heap/TraceTraits.h	2018-03-13 16:45:49.946244905 -0400
@@ -18,6 +18,7 @@
 #include "platform/wtf/HashTable.h"
 #include "platform/wtf/LinkedHashSet.h"
 #include "platform/wtf/ListHashSet.h"
+#include "platform/wtf/Optional.h"
 #include "platform/wtf/TypeTraits.h"
 
 namespace blink {
@@ -325,6 +326,23 @@ class TraceTrait<std::pair<T, U>> {
   }
 };
 
+// While using Optional<T> with garbage-collected types is generally disallowed
+// by the OptionalGarbageCollected check in blink_gc_plugin, garbage-collected
+// containers such as HeapVector are allowed and need to be traced.
+template <typename T>
+class TraceTrait<WTF::Optional<T>> {
+  STATIC_ONLY(TraceTrait);
+
+ public:
+  template <typename VisitorDispatcher>
+  static void Trace(VisitorDispatcher visitor, WTF::Optional<T>* optional) {
+    if (*optional != WTF::nullopt) {
+      TraceIfEnabled<T, WTF::IsTraceable<T>::value>::Trace(visitor,
+                                                           optional->value());
+    }
+  }
+};
+
 // If eager tracing leads to excessively deep |trace()| call chains (and
 // the system stack usage that this brings), the marker implementation will
 // switch to using an explicit mark stack. Recursive and deep object graphs
diff -up chromium-65.0.3325.146/third_party/WebKit/Source/platform/wtf/Optional.h.oilpan chromium-65.0.3325.146/third_party/WebKit/Source/platform/wtf/Optional.h
--- chromium-65.0.3325.146/third_party/WebKit/Source/platform/wtf/Optional.h.oilpan	2018-03-13 16:46:01.683015951 -0400
+++ chromium-65.0.3325.146/third_party/WebKit/Source/platform/wtf/Optional.h	2018-03-13 16:46:51.632043375 -0400
@@ -7,20 +7,15 @@
 
 #include "base/optional.h"
 #include "platform/wtf/TemplateUtil.h"
-#include "platform/wtf/TypeTraits.h"
 
 namespace WTF {
 
 // WTF::Optional is base::Optional. See base/optional.h for documentation.
 //
 // A clang plugin enforces that garbage collected types are not allocated
-// outside of the heap, similarly we enforce that one doesn't create garbage
-// collected types nested inside an Optional.
+// outside of the heap. GC containers such as HeapVector are allowed though.
 template <typename T>
-using Optional =
-    typename std::enable_if<!IsGarbageCollectedType<T>::value ||
-                                IsPersistentReferenceType<T>::value,
-                            base::Optional<T>>::type;
+using Optional = base::Optional<T>;
 
 constexpr base::nullopt_t nullopt = base::nullopt;
 constexpr base::in_place_t in_place = base::in_place;