Fix WebKit Win10 14393 tag to match updated Windows version.

BUG=828051

Change-Id: I68e2285c72c7722c8380d143d10a0c42e0c43877
Reviewed-on: https://chromium-review.googlesource.com/989669Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Jao-ke Chin-Lee <jchinlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547461}

testing/buildbot/chromium.webkit.json

@@ -305,7 +305,7 @@
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
             {
-              "os": "Windows-10-14393"
+              "os": "Windows-10-15063"
             }
           ],
           "shards": 6

testing/buildbot/test_suite_exceptions.pyl

@@ -3694,7 +3694,7 @@
         'swarming': {
           'dimension_sets': [
             {
-              'os': 'Windows-10-14393',
+              'os': 'Windows-10-15063',
             },
           ],
         },