Record UMA on number of days since data reduction proxy is enabled.

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 f88bbca1cdc651b8c4c1dc305195f6e397ead7fc..8c605e2fc3e7f7a2d039012df9436c65b4a00cbb 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -8379,6 +8379,18 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="DataReductionProxy.DaysSinceEnabled" units="days">
+  <owner>tbansal@chromium.org</owner>
+  <owner>bengr@chromium.org</owner>
+  <summary>
+    Records the number of days since data reduction proxy was last enabled by
+    the user. Recorded only if the data reduction proxy was last enabled since
+    this metric was added to Chromium (M-56). Recorded at Chrome startup if data
+    reduction proxy is already enabled, and when the user enables the data
+    reduction proxy.
+  </summary>
+</histogram>
+
 <histogram name="DataReductionProxy.EnabledState"
     enum="DataReductionProxyEnabledState">
   <owner>bengr@chromium.org</owner>