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Define and use mixins for the most common GPU bot configurations. To
avoid errors and differences between machines, in most of these mixins,
parameters like the OS have been flattened in.

Use these mixins on the chromium.gpu, chromium.gpu.fyi, and
client.v8.fyi waterfalls.

This CL is essentially a no-op; it only modifies a few timeouts due to
unification under the mixin "limited_capacity_bot".

Bug: 895815
Change-Id: Ia9da945d73720ea70ebb4983808eb23463ee1108
Reviewed-on: https://chromium-review.googlesource.com/c/1287431Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601324}