Add histogram to measure the read deferral on crosswebsite navigation

content/browser/loader/resource_loader.cc

Index: content/browser/loader/resource_loader.cc
diff --git a/content/browser/loader/resource_loader.cc b/content/browser/loader/resource_loader.cc
index 63195120388f9192f002504600611ac05d3dfe73..9ed51a38cbe6697657574f9a69b81ce611c362e7 100644
--- a/content/browser/loader/resource_loader.cc
+++ b/content/browser/loader/resource_loader.cc
@@ -6,6 +6,7 @@
 
 #include "base/command_line.h"
 #include "base/message_loop.h"
+#include "base/metrics/histogram.h"
 #include "base/time.h"
 #include "content/browser/child_process_security_policy_impl.h"
 #include "content/browser/loader/doomed_resource_handler.h"
@@ -213,6 +214,7 @@ void ResourceLoader::Init(scoped_ptr<net::URLRequest> request,
   delegate_ = delegate;
   last_upload_position_ = 0;
   waiting_for_upload_progress_ack_ = false;
+  compute_read_deferral_histogram_ = false;
   is_transferring_ = false;
   client_cert_store_ = client_cert_store.Pass();
 
@@ -362,8 +364,11 @@ void ResourceLoader::OnResponseStarted(net::URLRequest* unused) {
 
   CompleteResponseStarted();
 
-  if (is_deferred())
+  if (is_deferred()) {
+    compute_read_deferral_histogram_ = true;

[pasko, 2013/05/21 16:37:51]

I think reflecting the state rather than intention would be better:
"deferred_before_initial_resume_ = true;".

[James Simonsen, 2013/08/01 19:17:40]

I don't think you need this bool at all. Just check deferral_time_.is_null().

[clamy, 2013/08/05 14:33:44]

Done.

+    deferral_time_ = base::TimeTicks::Now();

[pasko, 2013/05/21 16:37:51]

this would sound more logical with the name response_started_time_

     return;
+  }
 
   if (request_->status().is_success()) {
     StartReading(false);  // Read the first chunk.
@@ -585,6 +590,11 @@ void ResourceLoader::StartReading(bool is_continuation) {
 void ResourceLoader::ResumeReading() {
   DCHECK(!is_deferred());
 
+  if (compute_read_deferral_histogram_) {
+    compute_read_deferral_histogram_ = false;
+    UMA_HISTOGRAM_TIMES("Net.ResourceLoader.ReadDeferral",

[pasko, 2013/05/21 16:37:51]

please also add a pairing change in:
tools/metrics/histograms/histograms.xml

Would require adding jar@ as reviewer for that part.

+                        base::TimeTicks::Now() - deferral_time_);
+  }
   if (request_->status().is_success()) {
     StartReading(false);  // Read the next chunk (OK to complete synchronously).
   } else {