Add histogram for key exchange strength

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 27455195e5988d3c44c62bad96bcef30c0a5c9a7..c7ec205133848f7fbdc861d79726d9c57f0727f2 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -24614,6 +24614,17 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Net.SSL_KeyExchange">
+  <owner>sigbjorn@opera.com</owner>
+  <owner>agl@chromium.org</owner>
+  <owner>rsleevi@chromium.org</owner>
+  <summary>
+    Key exchange information/strength, by connection method. Recorded for each
+    SSL/TLS connection in the socket pool where Connect() succeeds. See
+    |SSL_SESSION|'s key_exchange_info for more information.
+  </summary>
+</histogram>
+
 <histogram name="Net.SSLCertBlacklisted">
   <owner>agl@chromium.org</owner>
   <summary>
@@ -75420,6 +75431,21 @@ To add a new entry, add it with any value and run test to compute valid value.
   <affected-histogram name="Net.QuicSession.PacketReceived_Nack"/>
 </histogram_suffixes>
 
+<histogram_suffixes name="Net.SSL_KeyExchange_subsections" separator=".">
+  <owner>sigbjorn@opera.com</owner>
+  <owner>agl@chromium.org</owner>
+  <owner>rsleevi@chromium.org</owner>
+  <suffix name="DHE" label="DHE"/>
+  <suffix name="RSA" label="RSA"/>
+  <suffix name="ECDHE" label="ECDHE"/>
+  <suffix name="Unknown" label="Unknown"/>
+  <summary>
+    Key exchange information/strength, by connection method. Recorded for each
+    SSL/TLS connection in the socket pool where Connect() succeeds.

[davidben, 2015/09/15 14:35:35]

Given that these are all totally incomparable things, it doesn't seem there's
much point in grouping them together. You could just make them separate
histograms, which would also allow you to, for ECDHE, add a enum definition.

Likewise, there doesn't seem any sense in logging "Unknown" because values for
different key exchanges don't align. NOTREACHED() seems more appropriate.

[sigbjorn, 2015/09/15 16:30:25]

Done.

+  </summary>
+  <affected-histogram name="Net.SSL_KeyExchange"/>
+</histogram_suffixes>
+
 <histogram_suffixes name="NetConnectivity" separator=".">
   <suffix name="53.100B" label="100 bytes of data on port 53."/>
   <suffix name="53.100B.NoProxy"