Change chromium bots to trigger on chromium tree

Originally, we planned to have the chromium fyi bots only run for each
new openscreen commit, but against chromium HEAD.  The checked-in
gclient config for the chromium recipe isn't intended for that though,
as it was copied from v8's config.  This results in the bots not finding
a valid repo URL when they are triggered.

I believe the two ways to fix this are:
 1. Trigger on the chromium tree instead, so the bot is technically
    getting a chromium trigger, but then openscreen is set to HEAD by
    the current config in //s/s/r/chromium/gclient_config.py.
 2. Change the config in //s/s/r/chromium/gclient_config.py to rewrite
    the gclient solution for the current trigger.  It's unclear how to
    do this at the moment.

This change takes approach 1 since, in addition to being easier, it has
the added benefit of tracking our compile status in chromium without
openscreen even changing.

Bug: 951615
Change-Id: Ib34ffdf0ffb1f8abd5205b53348c93d8ba066194
Reviewed-on: https://chromium-review.googlesource.com/c/openscreen/+/1674390
Reviewed-by: Max Yakimakha <yakimakha@chromium.org>
Commit-Queue: Brandon Tolsch <btolsch@chromium.org>
diff --git a/infra/config/global/luci-scheduler.cfg b/infra/config/global/luci-scheduler.cfg
index fbe0d74..da09652 100644
--- a/infra/config/global/luci-scheduler.cfg
+++ b/infra/config/global/luci-scheduler.cfg
@@ -25,6 +25,16 @@
   triggers: "linux64_debug"
   triggers: "linux64_debug_gcc"
   triggers: "mac_debug"
+}
+
+trigger {
+  id: "chromium-trigger"
+  acl_sets: "default"
+  gitiles {
+    repo: "https://chromium.googlesource.com/chromium/src"
+    refs: "refs/heads/master"
+  }
+
   triggers: "chromium_linux64_debug"
   triggers: "chromium_mac_debug"
 }