Add an about:flag to support alternative data saver features

tools/metrics/histograms/enums.xml

Index: tools/metrics/histograms/enums.xml
diff --git a/tools/metrics/histograms/enums.xml b/tools/metrics/histograms/enums.xml
index 7d714ec713726ca82859c308a4c58decb259d696..0d7ff882cc87aeef8541965c19b294797383d550 100644
--- a/tools/metrics/histograms/enums.xml
+++ b/tools/metrics/histograms/enums.xml
@@ -21451,6 +21451,8 @@ uploading your change for review.  These are checked by presubmit scripts.
   <int value="-1254070521" label="enable-slimming-paint-invalidation"/>
   <int value="-1251411236" label="disable-new-md-input-view"/>
   <int value="-1246840031" label="OptInImeMenu:disabled"/>
+  <int value="-1242458403"
+      label="DataReductionProxyAlternativeBackEnd:enabled"/>
   <int value="-1241747717" label="enable-android-password-link"/>
   <int value="-1235586511" label="enable-datasaver-prompt"/>
   <int value="-1218608640" label="disable-offline-load-stale-cache"/>
@@ -22196,6 +22198,8 @@ from previous Chrome versions.
   <int value="1466380480" label="enable-device-discovery-notifications"/>
   <int value="1466502102" label="DelayNavigation:disabled"/>
   <int value="1469407485" label="disable-accelerated-2d-canvas"/>
+  <int value="1474994723"
+      label="DataReductionProxyAlternativeBackEnd:disabled"/>
   <int value="1479248574" label="disable-voice-input"/>
   <int value="1481562816" label="disable-password-link"/>
   <int value="1486171015" label="disable-fill-on-account-select"/>