Add chrome_public_test_apk to android-marshmallow-arm64-rel at 50% experiment.

Capacity's been refreshed. Let's see if we got enough.

Bug: 877280, 731759
Change-Id: Iadf72d1220133710c618bbb5468d4a30facf6955
Reviewed-on: https://chromium-review.googlesource.com/c/1306843
Reviewed-by: John Budorick <[email protected]>
Commit-Queue: Ben Pastene <[email protected]>
Cr-Commit-Position: refs/heads/master@{#603745}
diff --git a/testing/buildbot/test_suite_exceptions.pyl b/testing/buildbot/test_suite_exceptions.pyl
index fe9065c..78d7f97 100644
--- a/testing/buildbot/test_suite_exceptions.pyl
+++ b/testing/buildbot/test_suite_exceptions.pyl
@@ -221,10 +221,6 @@
     ],
   },
   'chrome_public_test_apk': {
-    'remove_from': [
-      # TODO(crbug.com/877280): Add it back when we have the capacity.
-      'android-marshmallow-arm64-rel',
-    ],
     'modifications': {
       # chromium.android
       'android-kitkat-arm-rel': {
@@ -232,6 +228,11 @@
           'shards': 12,
         },
       },
+      'android-marshmallow-arm64-rel': {
+        # TODO(crbug.com/731759): Enable this once we're confident that it
+        # passes.
+        'experiment_percentage': 50,
+      },
       'KitKat Phone Tester (dbg)': {
         'swarming': {
           'shards': 20,