Add android-pie-arm64-rel as experimental CQ builder.

Bug: 765833
Change-Id: I91b69339b11225dd52750f6d71aca986f508b472
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1762704
Commit-Queue: Andrew Luo <aluo@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#694985}

docs/infra/cq_builders.md

@@ -304,6 +304,10 @@ 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.
 
+* [android-pie-arm64-rel](https://ci.chromium.org/p/chromium/builders/luci.chromium.try/android-pie-arm64-rel) ([`commit-queue.cfg` entry](https://cs.chromium.org/search/?q=package:%5Echromium$+file:commit-queue.cfg+chromium/try/android-pie-arm64-rel)) ([matching builders](https://cs.chromium.org/search/?q=file:trybots.py+android-pie-arm64-rel))
+
+  * Experimental percentage: 2
+
 * [chromeos-kevin-experimental-rel](https://ci.chromium.org/p/chromium/builders/luci.chromium.try/chromeos-kevin-experimental-rel) ([`commit-queue.cfg` entry](https://cs.chromium.org/search/?q=package:%5Echromium$+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

infra/config/commit-queue.cfg

@@ -355,6 +355,10 @@ config_groups {
       # Experimental builders. #
       ##########################
 
+      builders {
+        name: "chromium/try/android-pie-arm64-rel"
+        experiment_percentage: 2
+      }
       builders {
         name: "chromium/try/chromeos-kevin-experimental-rel"
         experiment_percentage: 5