Revert "Stop running webkit_layout_tests on mac 10.12 and 10.13 w/ GPUs."

This reverts commit e64c72b9.

Reason for revert: This causes the https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Mac10.12%20Tests bot to turn red with webkit_layout_tests failures that are empty.

Original change's description:
> Stop running webkit_layout_tests on mac 10.12 and 10.13 w/ GPUs.
>
> And disable http/tests/media/video-load-metadata-decode-error.html,
> which appears to have a dependency on GPUs.
>
> Bug: 854874, 871445
> Change-Id: I254ce21953ac428351203d6479e1936347ce5b4c
> Reviewed-on: https://chromium-review.googlesource.com/1162963
> Commit-Queue: Ben Pastene <bpastene@chromium.org>
> Reviewed-by: Kenneth Russell <kbr@chromium.org>
> Reviewed-by: Dirk Pranke <dpranke@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#581074}

TBR=dpranke@chromium.org,kbr@chromium.org,bpastene@chromium.org

No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 854874, 871445, 871830
Change-Id: Ia9963a9f3cb6dbaad0f7a8b78723bf0dc034d052
Reviewed-on: https://chromium-review.googlesource.com/1165846
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581297}

testing/buildbot/chromium.mac.json

@@ -2266,7 +2266,7 @@
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
             {
-              "gpu": "none",
+              "gpu": "8086:0a2e",
               "os": "Mac-10.12.6"
             }
           ],
@@ -2843,7 +2843,7 @@
       {
         "alternate_swarming_dimensions": [
           {
-            "gpu": "none",
+            "gpu": "8086:0a2e",
             "os": "Mac-10.12.6"
           }
         ],
@@ -2860,7 +2860,7 @@
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
             {
-              "gpu": "none",
+              "gpu": "8086:0a2e",
               "os": "Mac-10.13"
             }
           ],

testing/buildbot/test_suite_exceptions.pyl

@@ -1644,7 +1644,7 @@
         'swarming': {
           'dimension_sets': [
             {
-              'gpu': 'none',
+              'gpu': '8086:0a2e',
               'os': 'Mac-10.12.6',
             },
           ],
@@ -1655,7 +1655,7 @@
         # TODO(crbug.com/853356): Switch this to 10.13.
         'use_multi_dimension_trigger_script': True,
         'alternate_swarming_dimensions': [{
-          'gpu': 'none',
+          'gpu': '8086:0a2e',
           'os': 'Mac-10.12.6',
         }],
 
@@ -1664,7 +1664,7 @@
         'swarming': {
           'dimension_sets': [
             {
-              'gpu': 'none',
+              'gpu': '8086:0a2e',
               'os': 'Mac-10.13',
             },
           ],

third_party/WebKit/LayoutTests/TestExpectations

@@ -4856,9 +4856,6 @@ crbug.com/871578 [ Mac ] external/wpt/xhr/timeout-multiple-fetches.html [ Failur
 crbug.com/871578 [ Mac ] virtual/outofblink-cors/external/wpt/xhr/timeout-multiple-fetches.html [ Failure Pass ]
 crbug.com/871578 [ Mac ] virtual/outofblink-cors-ns/external/wpt/xhr/timeout-multiple-fetches.html [ Failure Pass ]
 
-# Only passes on bots with GPUs.
-crbug.com/871445 [ Mac ] http/tests/media/video-load-metadata-decode-error.html [ Timeout ]
-
 # Flaky on various bots
 crbug.com/869470 external/wpt/background-fetch/fetch.https.window.html [ Pass Failure ]
 crbug.com/869470 external/wpt/background-fetch/get.https.window.html [ Pass Failure ]