Add missing "origin" field to test fixtures

Without this field all the test cases still pass, but for the wrong
reason, since the origin field is checked before other event properties.

src/sidebar/util/test/postmessage-json-rpc-test.js

@@ -70,17 +70,21 @@ describe('sidebar.util.postmessage-json-rpc', () => {
       },
     },{
       // Non-object `data` field.
+      origin,
       data: null,
     },{
       // No jsonrpc header
+      origin,
       data: {},
     },{
       // No ID
+      origin,
       data: {
         jsonrpc: '2.0',
       },
     },{
       // ID mismatch
+      origin,
       data: {
         jsonrpc: '2.0',
         id: 'wrong-id',