[V8] Switch layout tests to Xenial

Bug: 954890
Change-Id: I7b41df998f1ccfc72b597a70d8214850845f8c47
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1707281Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#678669}

testing/buildbot/client.v8.fyi.json

@@ -1297,7 +1297,7 @@
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
             {
-              "os": "Ubuntu-14.04"
+              "os": "Ubuntu-16.04"
             }
           ],
           "hard_timeout": 900
@@ -1323,7 +1323,7 @@
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
             {
-              "os": "Ubuntu-14.04"
+              "os": "Ubuntu-16.04"
             }
           ],
           "hard_timeout": 900,

testing/buildbot/waterfalls.pyl

@@ -4232,7 +4232,7 @@
         'swarming': {
           'dimension_sets': [
             {
-              'os': 'Ubuntu-14.04',
+              'os': 'Ubuntu-16.04',
             },
           ],
           'hard_timeout': 900,