Hide "root" variable in job web page

When displaying a job variant's variables, the web page shows
they are all descended from a variable named "root".  This is
misleading, and can be avoided by passing this parameter to
ReactJson.  This causes the root of the hierarchy to be an
unnamed dict.

Change-Id: I4f123507f21539221da47ebf5372a93b52ebf70b

web/src/containers/job/JobVariant.jsx

@@ -129,6 +129,7 @@ class JobVariant extends React.Component {
           <span style={{whiteSpace: 'pre'}}>
             <ReactJson
               src={value}
+              name={null}
               sortKeys={true}
               enableClipboard={false}
               displayDataTypes={false}/>