Update Mac FYI dEQP Release AMD to use 10.13.6.

Update "Mac FYI dEQP Release AMD" on the GPU FYI waterfall to look for
machines running OS 10.13.6.

Bug: chromium:881952
No-Try: True
Change-Id: I12e0f82f4cc29fdbe36f7f76ad5d945228eaf4a7
Reviewed-on: https://chromium-review.googlesource.com/1213704
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589685}

testing/buildbot/chromium.gpu.fyi.json

@@ -9729,7 +9729,7 @@
             {
               "gpu": "1002:6821",
               "hidpi": "1",
-              "os": "Mac-10.13.5",
+              "os": "Mac-10.13.6",
               "pool": "Chrome-GPU"
             }
           ],
@@ -9749,7 +9749,7 @@
             {
               "gpu": "1002:6821",
               "hidpi": "1",
-              "os": "Mac-10.13.5",
+              "os": "Mac-10.13.6",
               "pool": "Chrome-GPU"
             }
           ],
@@ -9769,7 +9769,7 @@
             {
               "gpu": "1002:6821",
               "hidpi": "1",
-              "os": "Mac-10.13.5",
+              "os": "Mac-10.13.6",
               "pool": "Chrome-GPU"
             }
           ],

testing/buildbot/waterfalls.pyl

@@ -1770,7 +1770,7 @@
             {
               'gpu': '1002:6821',
               'hidpi': '1',
-              'os': 'Mac-10.13.5',
+              'os': 'Mac-10.13.6',
               'pool': 'Chrome-GPU',
             },
           ],