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/1104796


Reviewed-by: Ryan Tseng <hinoka@chromium.org>
Commit-Queue: Roberto Carrillo <robertocn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568131}