Refactor diagnostic spans to share a polymer behavior.

tracing/tracing/value/ui/diagnostic_span.html

Index: tracing/tracing/value/ui/diagnostic_span.html
diff --git a/tracing/tracing/value/ui/diagnostic_span.html b/tracing/tracing/value/ui/diagnostic_span.html
index e0812f24b1c3236f4ea8aca972531474f8978b93..86c5547c99fa4ecfb60dd7fe3b137d9803c8ad89 100644
--- a/tracing/tracing/value/ui/diagnostic_span.html
+++ b/tracing/tracing/value/ui/diagnostic_span.html
@@ -61,14 +61,15 @@ tr.exportTo('tr.v.ui', function() {
    * Create a visualization for |diagnostic|.
    *
    * @param {!tr.v.d.Diagnostic} diagnostic
-   * @param {!tr.v.Histogram=} opt_histogram
+   * @param {string} name
+   * @param {!tr.v.Histogram} histogram
    * @return {Element}
    */
-  function createDiagnosticSpan(diagnostic, opt_histogram) {
+  function createDiagnosticSpan(diagnostic, name, histogram) {
     const tagName = findElementNameForDiagnostic(diagnostic);
     const span = document.createElement(tagName);
-    span.histogram = opt_histogram;
-    span.diagnostic = diagnostic;
+    if (span.build === undefined) throw new Error(tagName);
+    span.build(diagnostic, name, histogram);
     return span;
   }