Reland: Stop only running webkit_layout_tests on mac 10.13 w/ GPUs.

And disable http/tests/media/video-load-metadata-decode-error.html,
which appears to have a dependency on GPUs.

Fixes to original change:
- Instead of targeting bots w/o GPUs, this instead targets any 10.12 bot (GPUs or not).
- Doesn't change the suite on the Mac10.12 builder since all 10.12 bots have GPUs:
https://chromium.googlesource.com/chromium/src/+/2021963d6d4b6204fb4c935f487631e577ffc6ae/testing/buildbot/waterfalls.pyl#2892

Bug: 854874, 871445
Change-Id: Idf3603785500fe3aeee3f978e4e4f7ec18c76ba4
Reviewed-on: https://chromium-review.googlesource.com/1166338Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581724}

testing/buildbot/chromium.mac.json

@@ -2843,7 +2843,6 @@
       {
         "alternate_swarming_dimensions": [
           {
-            "gpu": "8086:0a2e",
             "os": "Mac-10.12.6"
           }
         ],
@@ -2860,7 +2859,6 @@
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
             {
-              "gpu": "8086:0a2e",
               "os": "Mac-10.13"
             }
           ],

testing/buildbot/test_suite_exceptions.pyl

@@ -1649,7 +1649,6 @@
         # TODO(crbug.com/853356): Switch this to 10.13.
         'use_multi_dimension_trigger_script': True,
         'alternate_swarming_dimensions': [{
-          'gpu': '8086:0a2e',
           'os': 'Mac-10.12.6',
         }],
 
@@ -1658,7 +1657,6 @@
         'swarming': {
           'dimension_sets': [
             {
-              'gpu': '8086:0a2e',
               'os': 'Mac-10.13',
             },
           ],

third_party/WebKit/LayoutTests/TestExpectations

@@ -4876,3 +4876,6 @@ crbug.com/871105 [ Linux ] svg/dom/remove-use-target-element-indirectly.html [ P
 crbug.com/872025 [ Mac Linux ] fast/css-grid-layout/crash-large-positions.html [ Pass Timeout ]
 crbug.com/871416 fast/spatial-navigation/snav-stay-in-overflow-div.html [ Pass Failure ]
 crbug.com/872242 [ Mac ] fast/events/autoscroll-over-scrollbar.html [ Pass Failure ]
+
+# Only passes on bots with GPUs.
+crbug.com/871445 [ Mac ] http/tests/media/video-load-metadata-decode-error.html [ Timeout ]