Measure browser->render->browser hop time in UMA.

Measure AsyncResourceHandler browser->render->browser hop time in UMA to
understand the potential performance benefits of handling redirects entiely in
the browser process instead.

BUG=395552

Review URL: https://codereview.chromium.org/555913002

Cr-Commit-Position: refs/heads/master@{#294165}

content/browser/loader/async_resource_handler.cc

@@ -14,6 +14,7 @@
 #include "base/memory/shared_memory.h"
 #include "base/metrics/histogram.h"
 #include "base/strings/string_number_conversions.h"
+#include "base/time/time.h"
 #include "content/browser/devtools/devtools_netlog_observer.h"
 #include "content/browser/host_zoom_map_impl.h"
 #include "content/browser/loader/resource_buffer.h"
@@ -113,6 +114,13 @@ void AsyncResourceHandler::OnFollowRedirect(int request_id) {
     return;
   }
 
+  if (!redirect_start_time_.is_null()) {
+    UMA_HISTOGRAM_TIMES("Net.AsyncResourceHandler_RedirectHopTime",
+                        TimeTicks::Now() - redirect_start_time_);
+    // Reset start time.
+    redirect_start_time_ = TimeTicks();
+  }
+
   ResumeIfDeferred();
 }
 
@@ -143,6 +151,8 @@ bool AsyncResourceHandler::OnRequestRedirected(
   if (!info->filter())
     return false;
 
+  redirect_start_time_ = TimeTicks::Now();
+
   *defer = did_defer_ = true;
   OnDefer();
 

content/browser/loader/async_resource_handler.h

@@ -8,6 +8,7 @@
 #include <string>
 
 #include "base/memory/ref_counted.h"
+#include "base/time/time.h"
 #include "content/browser/loader/resource_handler.h"
 #include "content/browser/loader/resource_message_delegate.h"
 #include "url/gurl.h"
@@ -78,6 +79,8 @@ class AsyncResourceHandler : public ResourceHandler,
 
   int64_t reported_transfer_size_;
 
+  base::TimeTicks redirect_start_time_;
+
   DISALLOW_COPY_AND_ASSIGN(AsyncResourceHandler);
 };
 

tools/metrics/histograms/histograms.xml

@@ -14102,6 +14102,15 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="Net.AsyncResourceHandler_RedirectHopTime" units="milliseconds">
+  <owner>bnc@chromium.org</owner>
+  <summary>
+    The time between the call of AsyncResourceHandler::OnRequestRedirected and
+    the FollowRedirect IPC message from ResourceHost, that is, the length of the
+    redirect browser-renderer-browser hop.
+  </summary>
+</histogram>
+
 <histogram name="Net.AsyncResourceHandler_SharedIOBuffer_Alloc" units="bytes">
   <owner>Please list the metric's owners. Add more owner tags as needed.</owner>
   <summary>The size of a SharedIOBuffer allocation.</summary>