New builder to run findit/chromium/export_bot_db periodically.

This builder is meant to be used with a cron job on luci scheduler to
update the bot_db as a json file in google storage readable by Findit.
(Instead of having Findit attempt to parse the python files where these
configs are defined, under the chromium_tests recipe module).

R=hinoka,chanli,stgao,liaoyuke,lijeffrey,prasadv

Bug: 787918
Change-Id: I668eaef3511ff82795ff80a2650ecadd741cd861
Reviewed-on: https://chromium-review.googlesource.com/1141092Reviewed-by: Ryan Tseng <hinoka@chromium.org>
Commit-Queue: Roberto Carrillo <robertocn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575816}

infra/config/global/cr-buildbucket.cfg

@@ -954,6 +954,17 @@ buckets {
       mixins: "linux"
     }
 
+    builders {
+      name: "linux_chromium_bot_db_exporter"
+      mixins: "linux"
+      recipe {
+        name: "findit/chromium/export_bot_db"
+        properties: "gs_bucket:findit-for-me"
+        properties: "gs_object:bot_db.json"
+      }
+      service_account: "findit-builder@chops-service-accounts.iam.gserviceaccount.com"
+    }
+
     builders {
       name: "linux_chromium_component_updater"
       mixins: "linux-ci"