Use <ul> and <li> tags for the step time breakdown

tensorboard/plugins/profile/overview_page/overview-page.html

@@ -105,8 +105,10 @@
         <paper-card heading="Performance Summary">
           <div class="card-content">
             <p><b>Average step time</b> (lower is better): <b><span class="steptime-average" >[[_steptime_ms_average]] ms</span> </b> <i style="opacity:0.7">(standard deviation = <span>[[_steptime_ms_stddev]]</span> ms)</i></p>
-            <p>- Average infeed: <span>[[_infeed_ms_average]]</span> ms</p>
-            <p>- Average compute: <span>[[_compute_ms_average]]</span> ms</p>
+            <ul hidden$=[[!_infeed_ms_average]]>
+              <li>Average infeed: <span>[[_infeed_ms_average]]</span> ms</li>
+              <li>Average compute: <span>[[_compute_ms_average]]</span> ms</li>
+            </ul>
             <p><b>Host idle time</b> (lower is better): <span>[[_host_idle_time_percent]]</span></p>
             <p><b>TPU idle time</b> (lower is better): <span>[[_device_idle_time_percent]]</p>
             <p><b>Utilization of TPU Matrix Units</b> (higher is better): <span>[[_mxu_utilization_percent]]</span></p>
@@ -208,8 +210,8 @@
        _mxu_utilization_percent: { type: String },
        _steptime_ms_average: { type: String },
        _steptime_ms_stddev: { type: String },
-       _infeed_ms_average: { type: String },
-       _compute_ms_average: { type: String },
+       _infeed_ms_average: { type: String, value: '' },
+       _compute_ms_average: { type: String, value: '' },
        _top_ops_heading: { type: String },
        _error_message: { type: String },
        _host_count: { type: String },