Drop browser_tests on mac_chromium_rel_ng to 50% experiment.

TBR=dpranke@chromium.org

No-Try: true
Bug: 828031
Change-Id: Ica36364071464f05fe96b52cf699672617b3bbfa
Reviewed-on: https://chromium-review.googlesource.com/1105414
Commit-Queue: John Budorick <jbudorick@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568228}

testing/buildbot/chromium.mac.json

@@ -2173,7 +2173,7 @@
           "--test-launcher-filter-file=../../testing/buildbot/filters/mac_window_server_killers.browser_tests.filter",
           "--gtest_shuffle"
         ],
-        "experiment_percentage": 100,
+        "experiment_percentage": 50,
         "swarming": {
           "can_use_on_swarming_builders": true,
           "dimension_sets": [

testing/buildbot/test_suite_exceptions.pyl

@@ -269,7 +269,7 @@
           '--test-launcher-filter-file=../../testing/buildbot/filters/mac_window_server_killers.browser_tests.filter',
           '--gtest_shuffle',
         ],
-        'experiment_percentage': 100,
+        'experiment_percentage': 50,
         'swarming': {
           'dimension_sets': [
             {