Refactor Histogram relationship diagnostics.

tracing/tracing/value/ui/histogram_span_test.html

Index: tracing/tracing/value/ui/histogram_span_test.html
diff --git a/tracing/tracing/value/ui/histogram_span_test.html b/tracing/tracing/value/ui/histogram_span_test.html
index bef197c3a7bc3c7c4bfcad143cd55469b44f4281..de3561135ebe9035dde6f653787efbc84338f135 100644
--- a/tracing/tracing/value/ui/histogram_span_test.html
+++ b/tracing/tracing/value/ui/histogram_span_test.html
@@ -253,16 +253,12 @@ tr.b.unittest.testSuite(function() {
   test('breakdownUnit', async function() {
     const root = new tr.v.Histogram('root', tr.b.Unit.byName.sizeInBytes);
     const sampleBreakdown = new tr.v.d.Breakdown();
-    sampleBreakdown.set('x', 30 << 20);
-    sampleBreakdown.set('y', 70 << 20);
+    sampleBreakdown.set('x', 30 << 20, 'a');
+    sampleBreakdown.set('y', 70 << 20, 'b');
     root.addSample(100 << 20, {sampleBreakdown});
-    const rhb = new tr.v.d.RelatedHistogramBreakdown();
-    root.diagnostics.set('rhb', rhb);
     const aHist = new tr.v.Histogram('a', tr.b.Unit.byName.sizeInBytes);
-    rhb.set('a', aHist);
     aHist.addSample(10 << 20);
     const bHist = new tr.v.Histogram('b', tr.b.Unit.byName.sizeInBytes);
-    rhb.set('b', bHist);
     bHist.addSample(90 << 20);
     const span = document.createElement('tr-v-ui-histogram-span');
     this.addHTMLOutput(span);
@@ -294,7 +290,7 @@ tr.b.unittest.testSuite(function() {
           diagnostics: new Map([
             [tr.v.d.RESERVED_NAMES.BENCHMARKS, new tr.v.d.GenericSet([
               'system_health.common_desktop'])],
-            ['breakdown', new tr.v.d.RelatedHistogramBreakdown()],
+            ['breakdown', new tr.v.d.Breakdown()],
           ]),
         });
     this.addHTMLOutput(span);