WindowActivityWatcherTest is failing with --enable-features=SingleProcessMash.
It seems that Mash creates an additional metric, and tab_ukm_test_helper recognizes this as an error.
The new metric appears through hiding the WindowTreeHost:
#2 0x00000d20f9d4 ukm::TestUkmRecorder::AddEntry()
#3 0x7f71471cea0e ukm::DelegatingUkmRecorder::Delegate::AddEntry()
#4 0x7f71471ce915 ukm::DelegatingUkmRecorder::AddEntry()
#5 0x7f71471dd665 ukm::internal::UkmEntryBuilderBase::Record()
#6 0x00000bd117eb (anonymous namespace)::LogWindowMetricsUkmEntry()
#7 0x00000bd11e9e WindowActivityWatcher::BrowserWatcher::MaybeLogWindowMetricsUkmEntry()
#8 0x00000bd11587 WindowActivityWatcher::OnBrowserNoLongerActive()
#9 0x00000bc8028d BrowserList::NotifyBrowserNoLongerActive()
#10 0x00000c27f3c7 BrowserView::OnWidgetActivationChanged()
#11 0x7f71513997c3 views::Widget::OnNativeWidgetActivationChanged()
#12 0x7f71513d3914 views::DesktopNativeWidgetAura::HandleActivationChanged()
#13 0x7f7139745848 views::DesktopWindowTreeHostMus::OnActiveFocusClientChanged()
#14 0x7f7143ee0fe3 aura::FocusSynchronizer::SetActiveFocusClient()
#15 0x7f7139745bd3 views::DesktopWindowTreeHostMus::HideImpl()
#16 0x7f7143f83d1d aura::WindowTreeHost::Hide()
#17 0x7f71513d7a60 views::DesktopNativeWidgetAura::Hide()
#18 0x7f715139829d views::Widget::Hide()
Comment 1 by bugdroid1@chromium.org
, Oct 24