Add bug link for disabled mash ash unittest

Bug: 902171
Change-Id: I13860617a1531a98f87a5af1d342bd69ef781976
Reviewed-on: https://chromium-review.googlesource.com/c/1318841
Commit-Queue: Evan Stade <estade@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605924}

testing/buildbot/filters/chromeos.single_process_mash.ash_unittests.filter

-# These tests fail. https://crbug.com/646565
+# Virtual keyboard: these tests fail. https://crbug.com/646565
 -AppListPresenterDelegateTest.TapAppListWithVirtualKeyboardDismissesVirtualKeyboard/0
 -AppListPresenterDelegateTest.TapAppListWithVirtualKeyboardDismissesVirtualKeyboard/1
 -AshKeyboardControllerTest.*
@@ -14,7 +14,7 @@
 -WindowAnimationsTest.SlideOutAnimation
 -WorkspaceLayoutManagerSystemUiAreaTest.*
 
-# These tests crash. https://crbug.com/646565
+# Virtual keyboard: these tests crash. https://crbug.com/646565
 -LockContentsViewKeyboardUnitTest.AutoLayoutExtraSmallUsersListForKeyboard
 -LockContentsViewKeyboardUnitTest.AutoLayoutSmallUsersListForKeyboard
 -LockContentsViewKeyboardUnitTest.PinSubmitWithVirtualKeyboardShown
@@ -23,8 +23,6 @@
 -LockLayoutManagerTest.KeyboardBounds
 -LockWindowVirtualKeyboardTest.VirtualKeyboardDoesNotCoverAuthView
 -MagnificationControllerTest.TextfieldFocusedWithKeyboard
-# Crashes in ui::FakeContextFactory::GetLastCompositorFrame().
--NonClientFrameControllerMashTest.ContentRegionNotDrawnForClient
 -OverviewVirtualKeyboardTest.ToggleOverviewModeHidesVirtualKeyboard
 -OverviewVirtualKeyboardTest.ToggleOverviewModeDoesNotHideLockedVirtualKeyboard
 -ShelfLayoutManagerKeyboardTest.ShelfIgnoreWorkAreaChangeInNonStickyMode
@@ -61,3 +59,6 @@
 -VirtualKeyboardTest.HitTestBoundsAreResetWhenContainerTypeChanges
 -VirtualKeyboardTrayTest.PerformActionTogglesVirtualKeyboard
 -WorkspaceLayoutManagerKeyboardTest.IgnoreWorkAreaChangeinNonStickyMode
+
+# https://crbug.com/902171
+-NonClientFrameControllerMashTest.ContentRegionNotDrawnForClient