[testing] switch to use xenial for test from linux cfi builder

Test from this builder consumes 2~3 machine hour in chrome pool per
hour averagely.
I took the stats from
http://shortn/_EstLqOoMvH

I fixed CFI issue in crbug.com/982470

crashpad_tests is failed on trusty too crbug.com/985656

Bug: 954450
Change-Id: I2202444354f64c91363adf5c51230986d521ba84
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1708212
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#679484}
2 files changed