[testing] switch to use xenial for test from linux-ozone-rel builder

Test from this builder consumes less than 1.27(CQ) + 0.27(CI) machine hour in
chrome pool from p99 in recent 30 days.
I took the stats from
http://shortn/_vS3Df0uFD0

Bug: 954450
Change-Id: I626d6448788c6f449dce98c4ed735aa9915c6850
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1712603Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#680272}

testing/buildbot/chromium.linux.json

@@ -7400,7 +7400,7 @@
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
             {
-              "os": "Ubuntu-14.04"
+              "os": "Ubuntu-16.04"
             }
           ]
         },
@@ -7415,7 +7415,7 @@
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
             {
-              "os": "Ubuntu-14.04"
+              "os": "Ubuntu-16.04"
             }
           ]
         },

testing/buildbot/waterfalls.pyl

@@ -3414,7 +3414,7 @@
       },
       'linux-ozone-rel': {
         'mixins': [
-          'linux-trusty',
+          'linux-xenial',
         ],
         'additional_compile_targets': [
           'chrome',