[mb] Change default os dimension from trusty to xenial

Currently default os dimension seems to be specified in
chromium_swarming module.
https://cs.chromium.org/chromium/build/scripts/slave/recipe_modules/chromium_swarming/api.py?l=460&rcl=4134bbc385e6082a8b1ad3ebaa0fb9872e1d4915

So this does not change os dimension of swarming tests
triggered from bots.

Bug: 954450
Change-Id: I67f184fb125a15a2b8c9396bfa7c1d98c3080617
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1574876
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@{#684005}

tools/mb/mb.py

@@ -519,7 +519,7 @@ class MetaBuildWrapper(object):
     if self.platform == 'darwin':
       os_dim = ('os', 'Mac-10.13')
     elif self.platform.startswith('linux'):
-      os_dim = ('os', 'Ubuntu-14.04')
+      os_dim = ('os', 'Ubuntu-16.04')
     elif self.platform == 'win32':
       os_dim = ('os', 'Windows-10')
     else: