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It was making enough connections to exceed the total socket pool limit,
which is currently 256, and apparently large enough to cause timeouts on
some bots.  Switch to using the per-group limit instead - socket pool
tests themselves should make sure that ignore_limits ignores the global
pool limits as well, as there's no other way to exceed the per-group
limit, so just checking the per-group limit should be sufficient.

Bug: 820845
Change-Id: I2a661b535492c5bbca4e90710bd2d91e6115edfe
Reviewed-on: https://chromium-review.googlesource.com/964542
Commit-Queue: Matt Menke <mmenke@chromium.org>
Reviewed-by: Eric Roman <eroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544391}