[testing] use xenial for cronet test on linux

From
http://shortn/_EstLqOoMvH
these test uses very small trusty resources in chrome pool.
So I think this is safe to land.

Bug: 954450
Change-Id: I249be4bf0a5ce12ae6f39ad04a982aa39104bb32
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1708310
Reviewed-by: Yoshisato Yanagisawa <yyanagisawa@google.com>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#678623}