Add an experiment for cast_shell_android

Bug: 731983
Change-Id: Ic40dc83011157ea7f6d23d7be52c2b8571074334
Reviewed-on: https://chromium-review.googlesource.com/1058310Reviewed-by: Nodir Turakulov <nodir@chromium.org>
Commit-Queue: Quinten Yearsley <qyearsley@chromium.org>
Cr-Commit-Position: refs/heads/master@{#559085}

infra/config/branch/cq.cfg

@@ -67,7 +67,7 @@ verifiers {
       }
       builders {
         name: "cast_shell_android"
-        equivalent_to { bucket: "luci.chromium.try" percentage: 0 }
+        equivalent_to { bucket: "luci.chromium.try" percentage: 10 }
       }
       builders {
         name: "linux_android_rel_ng"