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For ease of migration from trusty to xenial, I have written simple
script to replace the os dimension.
It also adds mixin to make the builder use builderless bots.

Bug: 954450
Change-Id: Iccce46a96a5ca262523f5b09e6ce308126031c5e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1645532
Commit-Queue: Yoshisato Yanagisawa <yyanagisawa@chromium.org>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#666633}