Explicitly request 16.04 swarming bots for the Linux Xenial bot

There was a build step (site_per_process_webkit_layout_tests) on Linux Xenial
that for some reason was requesting Trusty swarming bots.  All of the other
steps were using Xenial bots.  This CL explicitly requests Xenial swarming bots
to fix this step.

BUG=789768
R=dpranke@chromium.org

Change-Id: Id9b33f0fd189f2286cf6dbb03cfd1553bafdf354
Reviewed-on: https://chromium-review.googlesource.com/920764
Commit-Queue: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#536927}

testing/buildbot/waterfalls.pyl

@@ -1058,6 +1058,13 @@
         },
       },
       'Linux Xenial': {
+        'swarming': {
+          'dimension_sets': [
+            {
+              'os': 'Ubuntu-16.04',
+            },
+          ],
+        },
         'additional_compile_targets': [
           'all',
         ],