QUIC - add new UMA histograms to help understand HPACK head of line blocking.

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 218a6430bddf39aea69ee11b416956658648b731..f0cc6ff127e524cf37f58304606545eb1196f9cb 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -28359,6 +28359,16 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Net.QuicHpackDecoderIndexedEntryAge" units="millisconds">

[Alexei Svitkine (slow), 2016/05/24 18:00:30]

Nit: units="ms"

Same below.

[Buck, 2016/05/24 18:15:06]

Done.

+  <owner>ckrasic@chromium.org</owner>
+  <summary>
+    For each indexed representation decoded, records the time since the
+    corresponding entry was added to the dynamic table.  This data is being
+    collected to help analyze a proposed solution to HPACK induced head of line
+    blocking.
+  </summary>
+</histogram>
+
 <histogram name="Net.QuicHpackDecompressionPercentage" units="%">
   <owner>ckrasic@chromium.org</owner>
   <summary>
@@ -28367,6 +28377,16 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Net.QuicHpackEncoderIndexedEntryAge" units="millisconds">
+  <owner>ckrasic@chromium.org</owner>
+  <summary>
+    For each indexed representation encoded, records the time since the
+    corresponding entry was added to the dynamic table.  This data is being
+    collected to help analyze a proposed solution to HPACK induced head of line
+    blocking.
+  </summary>
+</histogram>
+
 <histogram name="Net.QuicInchoateClientHelloReason"
     enum="QuicServerConfigState">
   <owner>rtenneti@chromium.org</owner>