Track double taps to launch the previous task.

Change-Id: Icd792ed4f508f8a8540a5cd2d12c3bdfd5827410
diff --git a/proto/src/metrics_constants.proto b/proto/src/metrics_constants.proto
index 49972d6..568edab 100644
--- a/proto/src/metrics_constants.proto
+++ b/proto/src/metrics_constants.proto
@@ -447,5 +447,8 @@
 
     // Logs the action the user takes when an app ANR'd.
     ACTION_APP_ANR = 317;
+
+    // Logged when a user double taps the overview button to launch the previous task
+    OVERVIEW_LAUNCH_PREVIOUS_TASK = 318;
   }
 }