mac: move browser_tests back to pool:Chrome at 50% experiment.

Will promote back to non-experimental once pool:Chrome-quarantine has
been folded back into pool:Chrome.

Bug: 828031
Change-Id: Icb4f7da5b0aab18ec951bb612acf8d3e7e155a27
Reviewed-on: https://chromium-review.googlesource.com/c/1302657Reviewed-by: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#603184}

testing/buildbot/chromium.mac.json

@@ -1253,13 +1253,13 @@
           "--test-launcher-filter-file=../../testing/buildbot/filters/mac_window_server_killers.browser_tests.filter",
           "--gtest_shuffle"
         ],
+        "experiment_percentage": 50,
         "swarming": {
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
             {
               "gpu": "none",
-              "os": "Mac-10.11",
-              "pool": "Chrome-quarantine"
+              "os": "Mac-10.11"
             }
           ],
           "shards": 10
@@ -2391,13 +2391,13 @@
           "--test-launcher-filter-file=../../testing/buildbot/filters/mac_window_server_killers.browser_tests.filter",
           "--gtest_shuffle"
         ],
+        "experiment_percentage": 50,
         "swarming": {
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
             {
-              "gpu": "none",
-              "os": "Mac-10.12.6",
-              "pool": "Chrome-quarantine"
+              "gpu": "8086:0a2e",
+              "os": "Mac-10.12.6"
             }
           ],
           "shards": 10
@@ -3614,13 +3614,13 @@
           "--test-launcher-filter-file=../../testing/buildbot/filters/mac_window_server_killers.browser_tests.filter",
           "--gtest_shuffle"
         ],
+        "experiment_percentage": 50,
         "swarming": {
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
             {
               "gpu": "none",
-              "os": "Mac-10.13",
-              "pool": "Chrome-quarantine"
+              "os": "Mac-10.13"
             }
           ],
           "shards": 10
@@ -4740,13 +4740,13 @@
           "--test-launcher-filter-file=../../testing/buildbot/filters/mac_window_server_killers.browser_tests.filter",
           "--gtest_shuffle"
         ],
+        "experiment_percentage": 50,
         "swarming": {
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
             {
               "gpu": "none",
-              "os": "Mac-10.13",
-              "pool": "Chrome-quarantine"
+              "os": "Mac-10.13"
             }
           ],
           "shards": 10

testing/buildbot/test_suite_exceptions.pyl

@@ -107,14 +107,7 @@
           '--test-launcher-filter-file=../../testing/buildbot/filters/mac_window_server_killers.browser_tests.filter',
           '--gtest_shuffle',
         ],
-        'swarming': {
-          'dimension_sets': [
-            {
-              'pool': 'Chrome-quarantine',
-              'gpu': 'none',
-            },
-          ],
-        },
+        'experiment_percentage': 50,
       },
       'Mac10.12 Tests': {
         # A subset of tests seem to cause WindowServer deaths on VMs.
@@ -123,14 +116,7 @@
           '--test-launcher-filter-file=../../testing/buildbot/filters/mac_window_server_killers.browser_tests.filter',
           '--gtest_shuffle',
         ],
-        'swarming': {
-          'dimension_sets': [
-            {
-              'pool': 'Chrome-quarantine',
-              'gpu': 'none',
-            },
-          ],
-        },
+        'experiment_percentage': 50,
       },
       'Mac10.13 Tests': {
         # A subset of tests seem to cause WindowServer deaths on VMs.
@@ -139,14 +125,7 @@
           '--test-launcher-filter-file=../../testing/buildbot/filters/mac_window_server_killers.browser_tests.filter',
           '--gtest_shuffle',
         ],
-        'swarming': {
-          'dimension_sets': [
-            {
-              'pool': 'Chrome-quarantine',
-              'gpu': 'none',
-            },
-          ],
-        },
+        'experiment_percentage': 50,
       },
       'Mac10.13 Tests (dbg)': {
         # A subset of tests seem to cause WindowServer deaths on VMs.
@@ -155,14 +134,7 @@
           '--test-launcher-filter-file=../../testing/buildbot/filters/mac_window_server_killers.browser_tests.filter',
           '--gtest_shuffle',
         ],
-        'swarming': {
-          'dimension_sets': [
-            {
-              'pool': 'Chrome-quarantine',
-              'gpu': 'none',
-            },
-          ],
-        },
+        'experiment_percentage': 50,
       },
       # chromium.memory
       'Linux ASan LSan Tests (1)': {