cros: Fix issue with window being black if minimized before tablet mode.

Issue was:
1) Open two windows in clamshell, minimize both.
2) Enter tablet.
3) Activate both windows, second window will be a black screen.

It seems minimizing in clamshell mode sets opacity to 0. So force opacity
to 1 before activate to ensure visiblity.

Test: manual
Bug: 920463
Change-Id: I3858ca4ac771d23879678f8705b73a281df0b8ac
Reviewed-on: https://chromium-review.googlesource.com/c/1406243Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622171}

ash/app_list/home_launcher_gesture_handler.cc

@@ -698,7 +698,10 @@ bool HomeLauncherGestureHandler::SetUpWindows(Mode mode, aura::Window* window) {
   if (window && !windows.empty() && windows[0] != window &&
       windows[0]->IsVisible()) {
     // Do not run slide down animation for the |window| if another active
-    // window in mru list exists.
+    // window in mru list exists. Windows minimized in clamshell mode may
+    // have opacity of 0, so set them to 1 to ensure visibility.
+    if (wm::GetWindowState(window)->IsMinimized())
+      window->layer()->SetOpacity(1.f);
     window1_.reset();
     return false;
   }