Change mixin to include mastername property.

The component mapping updater needs the mastername property to be
defined. This mixin takes care of that.

R=hinoka

Bug: 803310
Change-Id: I579375231325cbc89ce31edef16e505eb16c0ccd
Reviewed-on: https://chromium-review.googlesource.com/1104796Reviewed-by: Ryan Tseng <hinoka@chromium.org>
Commit-Queue: Roberto Carrillo <robertocn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568131}

infra/config/global/cr-buildbucket.cfg

@@ -910,7 +910,7 @@ buckets {
 
     builders {
       name: "linux_chromium_component_updater"
-      mixins: "linux"
+      mixins: "linux-ci"
       recipe {
         name: "findit/chromium/update_components"
       }