Use xenial bot to run v8 tests

But does not change to xenial for test using armv7l cpu.

Bug: chromium:954890
Change-Id: I02268f469001f197210cde9c63804a3dcea06a7f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1687831
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62531}

tools/mb/mb.py

@@ -403,7 +403,7 @@ class MetaBuildWrapper(object):
     if self.platform == 'darwin':
       os_dim = ('os', 'Mac-10.12')
     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: