Add src-side configs for chromeos-kevin-experimental-rel trybot.

Bug: 989314
Change-Id: Idbef0fd0d7fcfb3058b45b8738da41a467b15aa9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1728317Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#682817}

docs/infra/cq_builders.md

@@ -296,6 +296,10 @@ These builders are run on some percentage of builds. Their results are ignored
 by CQ. These are often used to test new configurations before they are added
 as required builders.
 
+* [chromeos-kevin-experimental-rel](https://ci.chromium.org/p/chromium/builders/luci.chromium.try/chromeos-kevin-experimental-rel) ([`commit-queue.cfg` entry](https://cs.chromium.org/search/?q=package:%5Echromium$+file:commit-queue.cfg+chromium/try/chromeos-kevin-experimental-rel)) ([matching builders](https://cs.chromium.org/search/?q=file:trybots.py+chromeos-kevin-experimental-rel))
+
+  * Experimental percentage: 5
+
 * [ios-device](https://ci.chromium.org/p/chromium/builders/luci.chromium.try/ios-device) ([`commit-queue.cfg` entry](https://cs.chromium.org/search/?q=package:%5Echromium$+file:commit-queue.cfg+chromium/try/ios-device)) ([matching builders](https://cs.chromium.org/search/?q=file:trybots.py+ios-device))
 
   https://crbug.com/739556; make this non-experimental ASAP.

infra/config/commit-queue.cfg

@@ -342,6 +342,10 @@ config_groups {
       # Experimental builders. #
       ##########################
 
+      builders {
+        name: "chromium/try/chromeos-kevin-experimental-rel"
+        experiment_percentage: 5
+      }
       # https://crbug.com/739556; make this non-experimental ASAP.
       builders {
         name: "chromium/try/ios-device"

infra/config/cr-buildbucket.cfg

@@ -3905,6 +3905,14 @@ buckets {
       name: "chromeos-kevin-compile-rel"
       mixins: "builderless"
     }
+    # Identical to chromeos-kevin-rel, but runs experimentally on a fraction of
+    # all CLs. Separate from chromeos-kevin-rel due to different triggering
+    # policies.
+    builders {
+      mixins: "chromeos-try"
+      name: "chromeos-kevin-experimental-rel"
+      mixins: "builderless"
+    }
     builders {
       mixins: "chromeos-try"
       name: "chromeos-kevin-rel"

infra/config/luci-milo.cfg

@@ -4781,6 +4781,12 @@ consoles {
   builders {
     name: "buildbucket/luci.chromium.try/chromeos-arm-generic-rel"
   }
+  builders {
+    name: "buildbucket/luci.chromium.try/chromeos-kevin-compile-rel"
+  }
+  builders {
+    name: "buildbucket/luci.chromium.try/chromeos-kevin-experimental-rel"
+  }
   builders {
     name: "buildbucket/luci.chromium.try/chromeos-kevin-rel"
   }

tools/mb/mb_config.pyl

@@ -710,6 +710,7 @@
       'chromeos-amd64-generic-rel': 'cros_chrome_sdk_headless_ozone',
       'chromeos-arm-generic-rel': 'cros_chrome_sdk_dchecks_always_on',
       'chromeos-kevin-compile-rel': 'cros_chrome_sdk_headless_ozone',
+      'chromeos-kevin-experimental-rel': 'cros_chrome_sdk_headless_ozone',
       'chromeos-kevin-rel': 'cros_chrome_sdk_headless_ozone',
       'linux-chromeos-rel': 'chromeos_with_codecs_release_trybot',
       'linux-chromeos-compile-dbg': 'chromeos_with_codecs_debug_bot',