Reserving experiment proto field for per-experiment bug IDs.

BUG=

Review-Url: https://codereview.chromium.org/2558083004
Cr-Commit-Position: refs/heads/master@{#437559}

components/variations/proto/study.proto

@@ -44,7 +44,7 @@ message Study {
 
   // An experiment within the study.
   //
-  // Next tag: 13
+  // Next tag: 14
   message Experiment {
     // A named parameter value for this experiment.
     //