New metric to track usage of the new GAIA flow.

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 01963f4e74cdf4e458e63460de7f33f4c45d3792..df2e421966d5fd65a638bd48f69baf9cc261efaf 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -2782,6 +2782,14 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="ChromeOS.GAIA.WebViewFlow" enum="BooleanGAIAWebViewFlow">
+  <owner>ginkage@chromium.org</owner>
+  <summary>
+    Whether a user signed in using the new WebView-based GAIA flow. This value
+    is sent after the GAIA screen has completed user authentication.
+  </summary>
+</histogram>
+
 <histogram name="ChromeOS.MachineIdRegen.AgeSeconds" units="seconds">
   <owner>zeuthen@chromium.org</owner>
   <summary>
@@ -44992,6 +45000,11 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   <int value="1" label="Force Disabled"/>
 </enum>
 
+<enum name="BooleanGAIAWebViewFlow" type="int">
+  <int value="0" label="iframe-based flow"/>
+  <int value="1" label="WebView-based flow"/>
+</enum>
+
 <enum name="BooleanHadBlankText" type="int">
   <int value="0" label="Did not have blank text"/>
   <int value="1" label="Had blank text"/>