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The OnWindowActivated observer method does not guarantee that the
|gained_active| argument is non-null, so GuestOsEngagementMetrics should
not assume this.

This is causing tast tests to fail when the PluginVm feature is enabled,
as the matcher used, IsPluginVmWindow(), doesn't null-check the window.
ARC is unaffected as IsArcAppWindow() does a null-check.

Bug: 990252
Change-Id: I4673e1c9fbbc8142057481d51567be0e5677656d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1859417Reviewed-by: Nicholas Verne <nverne@chromium.org>
Commit-Queue: Timothy Loh <timloh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705476}