Switch trusty -> xenial for chromium_presubmit

This is step 2 written in
https://chrome-internal-review.googlesource.com/c/infradata/config/+/1304115

Bug: 954450
Change-Id: Ic23e80a9a28d83ed2d125c288369353eedfe6a67
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1613120
Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#660274}
1 file changed