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/1109458Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569147}

testing/buildbot/chromium.clang.json

@@ -16522,13 +16522,6 @@
         "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

@@ -789,22 +789,6 @@
           "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

@@ -2624,6 +2624,7 @@
         }
       },
       {
+        "experiment_percentage": 50,
         "isolate_name": "webkit_layout_tests_exparchive",
         "merge": {
           "args": [
@@ -2636,13 +2637,6 @@
         "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

@@ -2595,9 +2595,10 @@
   '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
+      'Linux Tests (dbg)(1)(32)', # 32-bit linux is unsupported by the layout tests.
       # chromium.win
       'Win 7 Tests x64 (1)',  # 64-bit win is unsupported by the layout tests.
       'Win10 Tests x64 (dbg)',
@@ -2760,41 +2761,14 @@
         },
       },
       'Mac10.13 Tests': {
-        # 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,
-        },
+        # TODO(crbug.com/853356): Ramp to 100% non-experimental.
+        'experiment_percentage': 50,
       },
       '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': {