File Manager: Enable TabIndex/FilesAppBrowserTest tests with Polymer 2.

Bug: 875525
Change-Id: I2bc1181d19b7bc3841da6d7306c85983f6656927
Reviewed-on: https://chromium-review.googlesource.com/1249903Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594895}

testing/buildbot/filters/webui_polymer2_browser_tests.filter

@@ -57,10 +57,6 @@
 -DemoSetupTest.OnlineSetupFlowSuccess
 -DemoSetupTest.ShowOfflineSetupOptionOnNetworkList
 -EulaTest.LoadOffline
--TabIndex/FilesAppBrowserTest.Test/tabindexSaveFileDialogDownloads
--TabIndex/FilesAppBrowserTest.Test/tabindexSaveFileDialogDownloads_GuestMode
--TabIndex/FilesAppBrowserTest.Test/tabindexSaveFileDialogDrive
--TabIndex/FilesAppBrowserTest.Test/tabindexSaveFileDialogDrive_DriveFs
 
 # Passing tests
 #
@@ -310,6 +306,7 @@ SettingsUIBrowserTest.*
 SigninSyncConfirmationTest.*
 SiteEngagementBrowserTest.*
 SysInternalsBrowserTest.*
+TabIndex/FilesAppBrowserTest.*
 TextDefaultsTest.*
 TtsAccessibilityTest.*
 UserManagerBrowserTest.*