Add metrics and histogram for IsolatedPrerender prefetching redirects

This also seems like a good metric to have, only recorded when
prefetching attempts have been made.

Bug: 1023485
Change-Id: I34c647d20c165386221c117e18f8212882fc9db2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2129891Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Commit-Queue: Robert Ogden <robertogden@chromium.org>
Cr-Commit-Position: refs/heads/master@{#755159}

chrome/browser/prerender/isolated/isolated_prerender_tab_helper.cc

@@ -114,6 +114,12 @@ void IsolatedPrerenderTabHelper::DidStartNavigation(
 
   // User is navigating, don't bother prefetching further.
   page_->url_loader_.reset();
+
+  if (page_->metrics_.prefetch_attempted_count > 0) {
+    UMA_HISTOGRAM_COUNTS_100(
+        "IsolatedPrerender.Prefetch.Mainframe.TotalRedirects",
+        page_->metrics_.prefetch_total_redirect_count);
+  }
 }
 
 void IsolatedPrerenderTabHelper::DidFinishNavigation(
@@ -253,6 +259,8 @@ void IsolatedPrerenderTabHelper::OnPrefetchRedirect(
     std::vector<std::string>* removed_headers) {
   DCHECK(PrefetchingActive());
 
+  page_->metrics_.prefetch_total_redirect_count++;
+
   // Run the new URL through all the eligibility checks. In the mean time,
   // continue on with other Prefetches.
   CheckAndMaybePrefetchURL(redirect_info.new_url);

chrome/browser/prerender/isolated/isolated_prerender_tab_helper.h

@@ -67,6 +67,9 @@ class IsolatedPrerenderTabHelper
     // The number of attempted prefetches that were successful (net error was OK
     // and HTTP response code was 2XX).
     size_t prefetch_successful_count = 0;
+
+    // The total number of redirects encountered during all prefetches.
+    size_t prefetch_total_redirect_count = 0;
   };
 
   const PrefetchMetrics& metrics() const { return page_->metrics_; }

tools/metrics/histograms/histograms.xml

@@ -67886,6 +67886,18 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
   </summary>
 </histogram>
 
+<histogram name="IsolatedPrerender.Prefetch.Mainframe.TotalRedirects"
+    units="count" expires_after="M90">
+  <owner>robertogden@chromium.org</owner>
+  <owner>ryansturm@chromium.org</owner>
+  <owner>tbansal@chromium.org</owner>
+  <summary>
+    Records the total number of redirects encountered while doing all the
+    prefetches on an eligible Google Search Result page. Only recorded when at
+    least one prefetch was attempted.
+  </summary>
+</histogram>
+
 <histogram name="IsolatedPrerender.Prefetch.Mainframe.TotalTime" units="ms"
     expires_after="M90">
   <owner>robertogden@chromium.org</owner>