[NoStatePrefetch] Fix the PrefetchAge histogram to match histograms.xml

chrome/browser/prerender/prerender_nostate_prefetch_browsertest.cc

Index: chrome/browser/prerender/prerender_nostate_prefetch_browsertest.cc
diff --git a/chrome/browser/prerender/prerender_nostate_prefetch_browsertest.cc b/chrome/browser/prerender/prerender_nostate_prefetch_browsertest.cc
index 522c0a49777e85c4681f992bb05de5ce7dbb9f24..69d7fb1f345c7bb17857366e9b996b84914ab639 100644
--- a/chrome/browser/prerender/prerender_nostate_prefetch_browsertest.cc
+++ b/chrome/browser/prerender/prerender_nostate_prefetch_browsertest.cc
@@ -178,6 +178,7 @@ IN_PROC_BROWSER_TEST_F(NoStatePrefetchBrowserTest, PrefetchHistograms) {
       "Prerender.websame_PrefetchTTFCP.Warm.Cacheable.Visible", 1);
   histogram_tester().ExpectTotalCount(
       "Prerender.websame_NoStatePrefetchResponseTypes", 2);
+  histogram_tester().ExpectTotalCount("Prerender.websame_PrefetchAge", 1);
 }
 
 // Checks the prefetch of an img tag.