[testing] switch to xenial for test from Mojo Linux builder

The builder has 25 hour hourly resource usage in p99.
http://shortn/_BO0EwXcF2a

So I expect this won't add much burden to existing xenial pool.
Current usage is in
https://datastudio.google.com/s/uf5Kh6NZ8XA
And we have 3250 xenial bots in MTV time now.

Bug: 954450
Change-Id: I85123c1b4c2eeabbf257739535a41679e4adf540
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1732704
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#683613}

testing/buildbot/chromium.fyi.json

@@ -901,7 +901,7 @@
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
             {
-              "os": "Ubuntu-14.04"
+              "os": "Ubuntu-16.04"
             }
           ],
           "shards": 15
@@ -921,7 +921,7 @@
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
             {
-              "os": "Ubuntu-14.04"
+              "os": "Ubuntu-16.04"
             }
           ],
           "shards": 15
@@ -941,7 +941,7 @@
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
             {
-              "os": "Ubuntu-14.04"
+              "os": "Ubuntu-16.04"
             }
           ],
           "shards": 15
@@ -961,7 +961,7 @@
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
             {
-              "os": "Ubuntu-14.04"
+              "os": "Ubuntu-16.04"
             }
           ]
         },
@@ -981,7 +981,7 @@
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
             {
-              "os": "Ubuntu-14.04"
+              "os": "Ubuntu-16.04"
             }
           ],
           "shards": 2
@@ -1001,7 +1001,7 @@
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
             {
-              "os": "Ubuntu-14.04"
+              "os": "Ubuntu-16.04"
             }
           ],
           "shards": 2
@@ -1021,7 +1021,7 @@
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
             {
-              "os": "Ubuntu-14.04"
+              "os": "Ubuntu-16.04"
             }
           ]
         },
@@ -1048,7 +1048,7 @@
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
             {
-              "os": "Ubuntu-14.04"
+              "os": "Ubuntu-16.04"
             }
           ],
           "shards": 12

testing/buildbot/waterfalls.pyl

@@ -1403,7 +1403,7 @@
       },
       'Mojo Linux': {
         'mixins': [
-          'linux-trusty',
+          'linux-xenial',
         ],
         'test_suites': {
           'gtest_tests': 'mojo_linux_gtests',