infra: make linux-win_cross-rel request os:Windows-10-15063.

Bug: 990885
Change-Id: I84a117cab1873469ad56115d955393f1111c7f66
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1779221
Commit-Queue: John Budorick <jbudorick@chromium.org>
Commit-Queue: Stephen Martinis <martiniss@chromium.org>
Auto-Submit: John Budorick <jbudorick@chromium.org>
Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#692185}

testing/buildbot/waterfalls.pyl

@@ -1019,13 +1019,9 @@
         },
       },
       'linux-win_cross-rel': {
-        'swarming': {
-          'dimension_sets': [
-            {
-              'os': 'Windows-10',
-            },
-          ],
-        },
+        'mixins': [
+          'win10',
+        ],
         'test_suites': {
           'gtest_tests': 'chromium_win_gtests',
         },