[testing] switch test to xenial for some linux builders

All of 4 builders have less than 6 hour hourly resource usage in p99.
http://shortn/_M8BzjMoQXa

So I expect this won't add much burden to existing xenial pool.

Bug: 954450
Change-Id: I0a6bae69557f8baa40e7abed1895025ded932dbc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1728416Reviewed-by: Yoshisato Yanagisawa <yyanagisawa@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#682640}

testing/buildbot/chromium.fyi.json

@@ -77,7 +77,7 @@
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
             {
-              "os": "Ubuntu-14.04"
+              "os": "Ubuntu-16.04"
             }
           ],
           "shards": 10
@@ -97,7 +97,7 @@
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
             {
-              "os": "Ubuntu-14.04"
+              "os": "Ubuntu-16.04"
             }
           ],
           "shards": 2
@@ -134,7 +134,7 @@
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
             {
-              "os": "Ubuntu-14.04"
+              "os": "Ubuntu-16.04"
             }
           ]
         }
@@ -168,7 +168,7 @@
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
             {
-              "os": "Ubuntu-14.04"
+              "os": "Ubuntu-16.04"
             }
           ]
         }
@@ -21699,7 +21699,7 @@
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
             {
-              "os": "Ubuntu-14.04"
+              "os": "Ubuntu-16.04"
             }
           ],
           "shards": 10

testing/buildbot/waterfalls.pyl

@@ -1303,7 +1303,7 @@
       },
       'Linux Viz': {
         'mixins': [
-          'linux-trusty',
+          'linux-xenial',
         ],
         'additional_compile_targets': [
           'all',
@@ -1830,7 +1830,7 @@
       },
       'linux-wpt-fyi-rel': {
         'mixins': [
-          'linux-trusty',
+          'linux-xenial',
         ],
         'test_suites': {
           'isolated_scripts': 'wpt_web_tests',
@@ -3278,7 +3278,7 @@
     'machines': {
       'Cast Audio Linux': {
         'mixins': [
-          'linux-trusty',
+          'linux-xenial',
         ],
         'additional_compile_targets': [
           'cast_shell',
@@ -3326,7 +3326,7 @@
       },
       'Leak Detection Linux': {
         'mixins': [
-          'linux-trusty',
+          'linux-xenial',
         ],
         'test_suites': {
           'isolated_scripts': 'leak_detection_isolated_scripts',