IPv6 related histograms.

Added histograms "Network.Shill.Cellular.IPv6ConnectivityStatus" and
"Network.Shill.Wifi.IPv6ConnectivityStatus" for tracking the IPv6
connectivity status when network connection is established. This
allows us to know the readiness of IPv6 connectivity in the field.

Added histograms "Network.Shill.Cellular.NetworkConnectionIPType" and
"Network.Shill.Wifi.NetworkConnectionIPType" for tracking the types of
IP configuration use for establishing network connections. This allows
us to track the usage of the newly added IPv6 connection support.

BUG=chromium:207192

Review URL: https://codereview.chromium.org/539163003

Cr-Commit-Position: refs/heads/master@{#293372}

tools/metrics/histograms/histograms.xml

@@ -18653,6 +18653,24 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="Network.Shill.Cellular.IPv6ConnectivityStatus"
+    enum="IPv6ConnectivityStatus">
+  <owner>zqiu@chromium.org</owner>
+  <summary>
+    Chrome OS network metric that tracks the presence of complete IPv6
+    configuration at the time when cellular connection is established.
+  </summary>
+</histogram>
+
+<histogram name="Network.Shill.Cellular.NetworkConnectionIPType"
+    enum="NetworkConnectionIPType">
+  <owner>zqiu@chromium.org</owner>
+  <summary>
+    Chrome OS network metric that tracks the types of IP configuration used for
+    establishing cellular connections.
+  </summary>
+</histogram>
+
 <histogram name="Network.Shill.Cellular.OutOfCreditsReason"
     enum="NetworkCellularOutOfCreditsReason">
   <owner>quiche@chromium.org</owner>
@@ -19219,6 +19237,15 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="Network.Shill.Wifi.IPv6ConnectivityStatus"
+    enum="IPv6ConnectivityStatus">
+  <owner>zqiu@chromium.org</owner>
+  <summary>
+    Chrome OS network metric that tracks the presence of complete IPv6
+    configuration at the time when WiFi connection is established.
+  </summary>
+</histogram>
+
 <histogram name="Network.Shill.Wifi.LinkMonitorBroadcastErrorsAtFailure">
   <owner>quiche@chromium.org</owner>
   <summary>
@@ -19264,6 +19291,15 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="Network.Shill.Wifi.NetworkConnectionIPType"
+    enum="NetworkConnectionIPType">
+  <owner>zqiu@chromium.org</owner>
+  <summary>
+    Chrome OS network metric that tracks the types of IP configuration used for
+    establishing WiFi connections.
+  </summary>
+</histogram>
+
 <histogram name="Network.Shill.Wifi.NetworkProblemDetected"
     enum="NetworkProblemType">
   <owner>zqiu@chromium.org</owner>
@@ -43412,6 +43448,11 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   <int value="1" label="GCMNetworkChannel"/>
 </enum>
 
+<enum name="IPv6ConnectivityStatus" type="int">
+  <int value="0" label="Incomplete IPv6 Configuration"/>
+  <int value="1" label="Complete IPv6 Configuration"/>
+</enum>
+
 <enum name="IPV6ProbeResult" type="int">
   <int value="0" label="IPV6_CANNOT_CREATE_SOCKETS"/>
   <int value="1" label="IPV6_CAN_CREATE_SOCKETS"/>
@@ -45960,6 +46001,11 @@ To add a new entry, add it with any value and run test to compute valid value.
   <int value="42" label="5230"/>
 </enum>
 
+<enum name="NetworkConnectionIPType" type="int">
+  <int value="0" label="IPv4"/>
+  <int value="1" label="IPv6"/>
+</enum>
+
 <enum name="NetworkCorruptedProfile" type="int">
   <int value="0" label="Corrupted Profile"/>
 </enum>