Increase Mac browser_tests experiment to 50%.

Bug: 828031
Change-Id: I395c4b33b7ac394645bf626c00185b4f9efcc289
Reviewed-on: https://chromium-review.googlesource.com/c/1294309Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601928}

testing/buildbot/chromium.mac.json

@@ -3614,7 +3614,7 @@
           "--test-launcher-filter-file=../../testing/buildbot/filters/mac_window_server_killers.browser_tests.filter",
           "--gtest_shuffle"
         ],
-        "experiment_percentage": 25,
+        "experiment_percentage": 50,
         "swarming": {
           "can_use_on_swarming_builders": true,
           "dimension_sets": [

testing/buildbot/test_suite_exceptions.pyl

@@ -139,7 +139,7 @@
           '--test-launcher-filter-file=../../testing/buildbot/filters/mac_window_server_killers.browser_tests.filter',
           '--gtest_shuffle',
         ],
-        'experiment_percentage': 25,
+        'experiment_percentage': 50,
         'swarming': {
           'dimension_sets': [
             {