Generalize default Win10 dimensions.

The Win10 GCE images were recently updated from RS1/14393 to RS2/15063.
Allow tasks triggered via mb to find machines on which to run.

Change-Id: I5a61f7d38d80cb441fa32a4bc9c0c7e23fbba75e
Reviewed-on: https://chromium-review.googlesource.com/1000721Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52488}

tools/mb/mb.py

@@ -389,7 +389,7 @@ class MetaBuildWrapper(object):
     elif self.platform.startswith('linux'):
       os_dim = ('os', 'Ubuntu-14.04')
     elif self.platform == 'win32':
-      os_dim = ('os', 'Windows-10-14393')
+      os_dim = ('os', 'Windows-10')
     else:
       raise MBErr('unrecognized platform string "%s"' % self.platform)