Add refs-tags-trigger to trigger android-cronet-x86-dbg builder only on tags.

Bug: 942176
Change-Id: Iaa4703b0d98693b9236c2e58a1449a91ba553d70
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1532915Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Misha Efimov <mef@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642742}

infra/config/luci-scheduler.cfg

@@ -295,7 +295,6 @@ trigger {
   triggers: "android-cronet-arm64-dbg"
   triggers: "android-cronet-arm64-rel"
   triggers: "android-cronet-asan-arm-rel"
-  triggers: "android-cronet-x86-dbg"
   triggers: "android-cronet-x86-rel"
   triggers: "android-dbg"
   triggers: "android-incremental-dbg"
@@ -397,6 +396,18 @@ trigger {
   triggers: "WebRTC Chromium FYI Win Builder (dbg)"
 }
 
+# These bots are triggered on new tags only.
+trigger {
+  id: "refs-tags-trigger"
+  acl_sets: "default"
+
+  gitiles: {
+    repo: "https://chromium.googlesource.com/chromium/src.git"
+    refs: "regexp:refs/tags/.+"
+  }
+
+  triggers: "android-cronet-x86-dbg"
+}
 
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 # Android Builders. Sorted alphabetically.