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Setting default RBE_PROD backend in some of the builder functions means
that any new builders added would use the RBE_PROD backend which might
overwhelm the backend if it is triggered often. Instead, require all
builders to be explicitly opted in to RBE_PROD backend.

The functions for the goma FYI builders were left as-is since they
incorporate the information about the backend in their names and its is
reasonable that the goma team would be specifically concerned with the
backend being set correctly to not accidentally add something using the
wrong backend.

Change-Id: Ia1ec7ec8d35232f02d503ecc4cec196f94a7f134
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1862259Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Garrett Beaty <gbeaty@chromium.org>
Cr-Commit-Position: refs/heads/master@{#706079}