Revert of Supporting study definitions without default groups and end_date filtering.

components/variations/proto/study.proto

Index: components/variations/proto/study.proto
diff --git a/components/variations/proto/study.proto b/components/variations/proto/study.proto
index 2b2f61162f17e855c2afcfbde031fb730ea8ab50..c97dbb4fff33771b20980b0b5a725761abf2886f 100644
--- a/components/variations/proto/study.proto
+++ b/components/variations/proto/study.proto
@@ -17,7 +17,6 @@
   // Ex: "my_study"
   required string name = 1;
 
-  // DEPRECATED: Prefer end_date instead.
   // The expiry date of the study in Unix time format. (Seconds since midnight
   // January 1, 1970 UTC). See: http://en.wikipedia.org/wiki/Unix_time
   //
@@ -39,8 +38,7 @@
   optional Consistency consistency = 7 [default = SESSION];
 
   // Name of the experiment that gets the default experience. This experiment
-  // must be included in the list below. If not specified, a generic default
-  // experiment name is used.
+  // must be included in the list below.
   // Ex: "default"
   optional string default_experiment_name = 8;
 
@@ -200,21 +198,12 @@
   // Filtering criteria specifying whether this study is applicable to a given
   // Chrome instance.
   //
-  // Next tag: 14
+  // Next tag: 13
   message Filter {
     // The start date of the study in Unix time format. (Seconds since midnight
     // January 1, 1970 UTC). See: http://en.wikipedia.org/wiki/Unix_time
     // Ex: 1330893974  (corresponds to 2012-03-04 20:46:14Z)
     optional int64 start_date = 1;
-
-    // The end date of the study in Unix time format. (Seconds since midnight
-    // January 1, 1970 UTC). See: http://en.wikipedia.org/wiki/Unix_time
-    // Ex: 1330893974  (corresponds to 2012-03-04 20:46:14Z)
-    // Mutually exclusive with expiry_date. The difference between end_date and
-    // expiry_date is that, when end_date is past, the field trial will not be
-    // created. When expiry_date is past, the trial is still created, but will
-    // be disabled, causing it to select its default group.
-    optional int64 end_date = 13;
 
     // The minimum Chrome version for this study, allowing a trailing '*'
     // character for pattern matching. Inclusive. (To check for a match, iterate