[test] Switch gcc testing to Xenial

NOTRY=true
TBR=sergiyb@chromium.org

Bug: chromium:933093
Change-Id: I48236ef06c990526b72be418773d0a098c85178f
Reviewed-on: https://chromium-review.googlesource.com/c/1488754Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59851}

infra/testing/builders.pyl

@@ -71,6 +71,9 @@
     ],
   },
   'v8_linux_gcc_rel': {
+    'swarming_dimensions' : {
+      'os': 'Ubuntu-16.04',
+    },
     'tests': [
       {'name': 'v8testing'},
     ],
@@ -729,6 +732,9 @@
     ],
   },
   'V8 Linux gcc 4.8': {
+    'swarming_dimensions' : {
+      'os': 'Ubuntu-16.04',
+    },
     'tests': [
       {'name': 'v8testing'},
     ],