Notify NQE of TCP RTT values

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index e909a2df2da07cd6a703971b40bf6290abf9e165..9c035b80b971eb74076fe873567760c118f48391 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -47323,6 +47323,18 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="SPW.TCPRTT.NotifyDelay" units="ms">
+  <owner>bengr@chromium.org</owner>
+  <owner>tbansal@chromium.org</owner>
+  <summary>
+    Time taken to read tcp_info struct through a syscall on POSIX platforms, and
+    notifying the RTT from the struct to the socket performance watcher.
+
+    Logged only on POSIX platforms when Chromium successfully read the RTT from
+    the tcp_info struct.

[Ryan Sleevi, 2016/02/25 22:48:47]

This entire histogram description seems wrong - because it means, based on your
documentation, you'll never log it on Windows, even if you implemented it on
Windows.

I'm sorry I wasn't clearer, but I don't think you have to do it via UMA. For
example

{
  tracked_objects::ScopedTracker tracking_profile(FROM_HERE);
  // ... Do stuff
}

Then you can use the existing jank infrastructure to measure this. The Chrome
perf folks can certainly help explain this, as this is how we quantified the
perf code yellow (and these metrics are uploaded to UMA, IIRC)

[tbansal1, 2016/02/26 23:43:34]

Thanks,.removed the UMA. Now, I am using the ScopedTracker.

+  </summary>
+</histogram>
+
 <histogram name="Sqlite.AppCache.Error" enum="SqliteErrorCode">
   <obsolete>
     Moved to Sqlite.Error.AppCache in M-27.