Histogram for FallbackDNSTestResult.

Added a histogram "Network.Shill.Wifi.FallbackDNSTestResult" for tracking
the result of the fallback DNS test. The fallback DNS test is performed
when portal detection is failed due to DNS failure.

BUG=chromium:366301
TEST=Start chrome, and browse to chrome://histograms to make sure
this histogram exist after connect to a wifi network without internet
access.

Review URL: https://codereview.chromium.org/291823003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@273110 0039d316-1c4b-4281-b951-d872f2087c98

tools/metrics/histograms/histograms.xml

@@ -16237,6 +16237,16 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="Network.Shill.Wifi.FallbackDNSTestResult"
+    enum="FallbackDNSTestResult">
+  <owner>zqiu@chromium.org</owner>
+  <summary>
+    Chrome OS network performance metric that tracks the result of the fallback
+    DNS test. The fallback DNS test is performed when portal detection failed
+    due to DNS failure.
+  </summary>
+</histogram>
+
 <histogram name="Network.Shill.WiFi.FrequenciesConnectedEver">
   <owner>quiche@chromium.org</owner>
   <summary>
@@ -35362,6 +35372,11 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   <int value="3" label="Unfused; lot ID NZVR7"/>
 </enum>
 
+<enum name="FallbackDNSTestResult" type="int">
+  <int value="0" label="Success"/>
+  <int value="1" label="Failure"/>
+</enum>
+
 <enum name="FallbackSSLVersion" type="int">
   <int value="0" label="FALLBACK_NONE">SSL version fallback did not occur.</int>
   <int value="1" label="FALLBACK_SSL3">Fell back on SSL 3.0.</int>