Edited branch-head config group.

Added comment and set linux-rel as 100% experimental
in the new config group.

Bug: 922150
Change-Id: I67e85580e8a43567d2189ee1cbaf578769065b8b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1769693
Commit-Queue: Karen Qian <karenqian@google.com>
Reviewed-by: Aaron Gable <agable@chromium.org>
Cr-Commit-Position: refs/heads/master@{#690923}

infra/config/commit-queue.cfg

@@ -374,6 +374,9 @@ config_groups {
   }
 }
 
+# Config group for tryjobs for branch-heads.
+# Will be removed when it has converged with
+# the refs/heads group.
 config_groups {
   gerrit {
     url: "https://chromium-review.googlesource.com"
@@ -391,11 +394,12 @@ config_groups {
       url: "https://chromium-status.appspot.com/"
     }
     tryjob {
-      #############################
-      # Always required builders. #
-      #############################
+      ###########################################
+      # Experimental builders for branch-heads. #
+      ###########################################
       builders {
         name: "chromium/try/linux-rel"
+        experiment_percentage: 100
       }
 
       retry_config {