SPM: Fix not painting after activating minimized windows

Minimizing a window triggers the following:
DesktopWindowTreeHostMus::OnWindowTreeHostWindowVisibilityChanged
=> WindowTreeHost::Hide()
=> Compositor::SetVisible(false)

Focusing on a minimized window implicitly unminimizing it. However,
Compositor::SetVisible(true) is not called hence the bug. This CL
adds the call to RestoreToPreminimizedState to fix the issue.

Bug: 942787
Change-Id: I37c7e99ebb01c8eb4cefefcf8c377b2c31c908fd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1533151Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642910}

ui/views/mus/desktop_window_tree_host_mus.cc

@@ -294,6 +294,8 @@ void DesktopWindowTreeHostMus::RestoreToPreminimizedState() {
   window_tree_host_window_observer_->set_is_waiting_for_restore(true);
   base::AutoReset<bool> setter(&is_updating_window_visibility_, true);
   window()->Show();
+  if (compositor())
+    compositor()->SetVisible(true);
 }
 
 void DesktopWindowTreeHostMus::OnWindowTreeHostWindowVisibilityChanged(

ui/views/mus/desktop_window_tree_host_mus_unittest.cc

@@ -735,6 +735,7 @@ TEST_F(DesktopWindowTreeHostMusTest, MinimizeActivate) {
   EXPECT_TRUE(widget->IsActive());
   EXPECT_TRUE(widget->IsVisible());
   EXPECT_FALSE(widget->IsMinimized());
+  EXPECT_TRUE(widget->GetNativeWindow()->GetHost()->compositor()->IsVisible());
 }
 
 TEST_F(DesktopWindowTreeHostMusTest, MaximizeMinimizeRestore) {