Add MobileMenuDataSaverOpened to actions.xml.

tools/metrics/actions/actions.xml

Index: tools/metrics/actions/actions.xml
diff --git a/tools/metrics/actions/actions.xml b/tools/metrics/actions/actions.xml
index 91a7f4545bebf889b1f6ec9fdb7a22a1a52c0c7c..6c07e16a15a15cbc6a5d01f2f42224d053912db3 100644
--- a/tools/metrics/actions/actions.xml
+++ b/tools/metrics/actions/actions.xml
@@ -9794,6 +9794,11 @@ should be able to be added at any place in this file.
   <obsolete>This menu item was never added to the app menu.</obsolete>
 </action>
 
+<action name="MobileMenuDataSaverOpened">
+  <owner>megjablon@chromium.org</owner>
+  <description>User pressed the Data Saver item in the app menu.</description>

[tapted, 2017/04/05 12:31:51]

I didn't add quotes because the item says "{size bytes} saved since {date}". LMK
if we want something else here.

+</action>
+
 <action name="MobileMenuDirectShare">
   <owner>aurimas@chromium.org</owner>
   <description>