scalars: commit chart changes in batch (#4053)

Summary:
In #3524, we defined a batch API for chart updates to avoid useless
paints that would be immediately overwritten. We now take further
advantage of this API to only paint charts once all data has been
loaded.

Test Plan:
On the hparams demo directory, this brings paint time (post-network)
for four charts rendering 50 trials each down from about 8 seconds to
under 1 second.

wchargin-branch: scalars-batch-commit

tensorboard/components_polymer3/tf_line_chart_data_loader/tf-line-chart-data-loader.ts

@@ -207,6 +207,7 @@ class _TfLineChartDataLoader<ScalarMetadata>
   _maybeRenderedInBadState: boolean = false;
 
   onLoadFinish() {
+    this.commitChanges();
     if (this.dataToLoad.length > 0 && this._resetDomainOnNextLoad) {
       // (Don't unset _resetDomainOnNextLoad when we didn't
       // load any runs: this has the effect that if all our

tensorboard/plugins/scalar/polymer3/tf_scalar_dashboard/tf-scalar-card.ts

@@ -238,7 +238,6 @@ export class TfScalarCard extends PolymerElement {
     const name = this._getSeriesNameFromDatum(item);
     scalarChart.setSeriesMetadata(name, item);
     scalarChart.setSeriesData(name, formattedData);
-    scalarChart.commitChanges();
   };
 
   @property({type: Object})