Switch interactive_ui_tests to experiment

This won't block CQ, but will let us see if a potential fix works.

Bug: 826735
Change-Id: I23c8e9a7b1df04206313df3fdbec58d733d17f7f
Reviewed-on: https://chromium-review.googlesource.com/986947Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Stephen Martinis <martiniss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547042}

testing/buildbot/chromium.win.json

@@ -892,6 +892,14 @@
         },
         "test": "installer_util_unittests"
       },
+      {
+        "experiment_percentage": 100,
+        "swarming": {
+          "can_use_on_swarming_builders": true,
+          "shards": 2
+        },
+        "test": "interactive_ui_tests"
+      },
       {
         "swarming": {
           "can_use_on_swarming_builders": true

testing/buildbot/test_suite_exceptions.pyl

@@ -1777,8 +1777,6 @@
       'Chromium Win 10 GCE Tests',
       # chromium.memory
       'Linux TSan Tests',
-      # chromium.win; temporary, https://crbug.com/826735
-      'Win10 Tests x64',
     ],
     'modifications': {
       # chromium.clang
@@ -1870,6 +1868,10 @@
           'shards': 3,
         },
       },
+      # chromium.win; temporary, https://crbug.com/826735
+      'Win10 Tests x64': {
+        'experiment_percentage': 100,
+      },
     },
   },
   'ipc_tests': {