Revert "Move the CQ Mac layout tests from 10.12 to 10.13."

This reverts commit 52f13686.

Reason for revert: I should just use the custom trigger script and dimension sets instead ...

Original change's description:
> Move the CQ Mac layout tests from 10.12 to 10.13.
> 
> Currently the Mac10.13 Tests bot runs the layout tests on 10.12;
> this is done because we didn't have enough capacity on 10.13 to
> run them. This CL is one of a series that is attempting to fix this.
> 
> In this particular CL, this switches the test step from 100%
> 10.12 to 50% 10.13, and also updates the chromium.clang ToTMac
> and 'Chromium Mac 10.13' bots to get around the unneeded hacks.
> 
> A subsequent CL will ramp this to 100% (non-experimental) once
> we're sure we have the capacity.
> 
> BUG=853356
> 
> Change-Id: I4a772f4bd82f427fde1931417dd60d0f80a8d40a
> Reviewed-on: https://chromium-review.googlesource.com/1109458
> Reviewed-by: John Budorick <jbudorick@chromium.org>
> Commit-Queue: Dirk Pranke <dpranke@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#569147}

TBR=dpranke@chromium.org,tikuta@chromium.org,jbudorick@chromium.org

Change-Id: I8cd37858ad7fe16a43009168d0d16a0535dac7e5
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 853356
Reviewed-on: https://chromium-review.googlesource.com/1108619Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569153}

testing/buildbot/chromium.clang.json

@@ -16522,6 +16522,13 @@
         "results_handler": "layout tests",
         "swarming": {
           "can_use_on_swarming_builders": true,
+          "dimension_sets": [
+            {
+              "gpu": "8086:0a2e",
+              "hidpi": "0",
+              "os": "Mac-10.12.6"
+            }
+          ],
           "shards": 12
         }
       },

testing/buildbot/chromium.fyi.json

@@ -789,6 +789,22 @@
           "can_use_on_swarming_builders": true
         }
       },
+      {
+        "isolate_name": "webkit_layout_tests_exparchive",
+        "merge": {
+          "args": [
+            "--verbose"
+          ],
+          "script": "//third_party/blink/tools/merge_web_test_results.py"
+        },
+        "name": "webkit_layout_tests",
+        "only_retry_failed_tests": true,
+        "results_handler": "layout tests",
+        "swarming": {
+          "can_use_on_swarming_builders": true,
+          "shards": 12
+        }
+      },
       {
         "isolate_name": "webkit_python_tests",
         "name": "webkit_python_tests",

testing/buildbot/chromium.mac.json

@@ -2624,7 +2624,6 @@
         }
       },
       {
-        "experiment_percentage": 50,
         "isolate_name": "webkit_layout_tests_exparchive",
         "merge": {
           "args": [
@@ -2637,6 +2636,13 @@
         "results_handler": "layout tests",
         "swarming": {
           "can_use_on_swarming_builders": true,
+          "dimension_sets": [
+            {
+              "gpu": "8086:0a2e",
+              "hidpi": "0",
+              "os": "Mac-10.12.6"
+            }
+          ],
           "shards": 12
         }
       },

testing/buildbot/test_suite_exceptions.pyl

@@ -2595,10 +2595,9 @@
   'webkit_layout_tests': {
     'remove_from': [
       # chromium.fyi
-      'Chromium Mac 10.13',  # no need to run this here.
       'Out of Process Profiling Mac',
       # chromium.linux
-      'Linux Tests (dbg)(1)(32)', # 32-bit linux is unsupported by the layout tests.
+      'Linux Tests (dbg)(1)(32)', # 32-bit linux is unsupported
       # chromium.win
       'Win 7 Tests x64 (1)',  # 64-bit win is unsupported by the layout tests.
       'Win10 Tests x64 (dbg)',
@@ -2761,14 +2760,41 @@
         },
       },
       'Mac10.13 Tests': {
-        # TODO(crbug.com/853356): Ramp to 100% non-experimental.
-        'experiment_percentage': 50,
+        # TODO(jbudorick,dpranke): Switch this to 10.13.
+
+        # TODO(kbr): if the Swarming dimensions were explicitly specified for
+        # all jobs then this wouldn't be needed. However, note that this bot
+        # implicitly specifies gpu:none rather than gpu:8086:0a2e.
+        'swarming': {
+          'dimension_sets': [
+            {
+              'gpu': '8086:0a2e',
+              'hidpi': '0',
+              'os': 'Mac-10.12.6',
+            },
+          ],
+          'shards': 12,
+        },
       },
       'Mac10.13 Tests (dbg)': {
         'args': [
           '--debug',
         ],
       },
+      # chromium.clang
+      'ToTMac': {
+        # TODO(thakis): Remove this once Mac10.13 Tests no longer runs tests on 10.12.
+        'swarming': {
+          'dimension_sets': [
+            {
+              'gpu': '8086:0a2e',
+              'hidpi': '0',
+              'os': 'Mac-10.12.6',
+            },
+          ],
+          'shards': 12,
+        },
+      },
 
       # chromium.webkit.
       'WebKit Linux Trusty ASAN': {