Fix SerializedBlobStructTraits.

The typemap defines this as nullable_is_same_type, but without a SetToNull
method that will result in runtime failures if null is actually passed.

Bug: none
Change-Id: Ife6427121c9f424ab8a2a0c52798f1806b650880
Reviewed-on: https://chromium-review.googlesource.com/956418Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Marijn Kruisselbrink <mek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542191}

third_party/WebKit/Source/platform/blob/SerializedBlobStructTraits.h

@@ -17,6 +17,14 @@ template <>
 struct PLATFORM_EXPORT
     StructTraits<blink::mojom::blink::SerializedBlob::DataView,
                  scoped_refptr<blink::BlobDataHandle>> {
+  static bool IsNull(const scoped_refptr<blink::BlobDataHandle>& input) {
+    return !input;
+  }
+
+  static void SetToNull(scoped_refptr<blink::BlobDataHandle>* output) {
+    *output = nullptr;
+  }
+
   static WTF::String uuid(const scoped_refptr<blink::BlobDataHandle>& input) {
     return input->Uuid();
   }