WebUI Polymer 2 migration: Re-enable interactive_ui_tests

These tests were disabled by https://crrev.com/c/1176206 because
they were reported as flaky on debug builds. Based on the
flakiness dashboard, it appears the failures were timeouts on the
Win10 Tests x64 (dbg) bot. interactive_ui_tests is marked as
experimental on this bot due to crbug.com/840369. The
webui_polymer2_interactive_ui_tests suite is expected to have the same
behavior, so it should also be marked as experimental on this bot.

Remove the tests from the blacklist, as they should pass on other bots.

Bug: 875443
Change-Id: Ib5b3c99cf589b1ab9994346460e95ed746533ba6
Reviewed-on: https://chromium-review.googlesource.com/1187729Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#586066}

testing/buildbot/chromium.win.json

@@ -1766,6 +1766,7 @@
           "--enable-features=WebUIPolymer2",
           "--test-launcher-filter-file=../../testing/buildbot/filters/webui_polymer2_interactive_ui_tests.filter"
         ],
+        "experiment_percentage": 100,
         "name": "webui_polymer2_interactive_ui_tests",
         "swarming": {
           "can_use_on_swarming_builders": true

testing/buildbot/filters/webui_polymer2_interactive_ui_tests.filter

@@ -22,15 +22,3 @@ SettingsUIBrowserTest.All
 -CrElementsActionMenuTest.All
 -CrElementsInputTest.All
 -MaterialBookmarksFocusTest.All
-
-# TODO(rbpotter): The following tests are flaky on debug builds. Fix them and
-# remove these exclusions.
--CrElementsCheckboxTest.All
--CrElementsProfileAvatarSelectorFocusTest.All
--CrElementsToggleTest.All
--CrSettingsAnimatedPagesTest.All
--CrSettingsFocusRowBehavior.FocusTest
--CrSettingsSyncPageTest.All
--MaterialHistoryFocusTest.All
--PrintPreviewDestinationDialogInteractiveTest.FocusSearchBox
--PrintPreviewPrintHeaderInteractiveTest.FocusPrintOnReady

testing/buildbot/test_suite_exceptions.pyl

@@ -1765,6 +1765,15 @@
       },
     },
   },
+  'webui_polymer2_interactive_ui_tests': {
+    'modifications': {
+      # Match interactive_ui_tests, see https://crbug.com/840369
+      # chromium.win
+      'Win10 Tests x64 (dbg)': {
+        'experiment_percentage': 100,
+      },
+    },
+  },
   'webview_instrumentation_test_apk': {
     'remove_from': [
       # This test frequently fails on Android, https://crbug.com/824959