Make notification view non-activatable on Chrome OS

This undoes the effect of http://crrev.com/c/1530962 on Chrome OS.

That CL has introduced a high frequency crash on Chrome OS in M75
(~1.5% of all browser process crashes). Its original intent was to
change behavior on Linux. Said behavior (ability to activate
notification views for inline replies) worked on Chrome OS before,
and continues working with that CL reverted.

Bug: 957110, 940924
Test: notification with inline reply works (see steps in bug 940924)
Change-Id: I971e46a7c22e942f5a8e54b0f64c5a08f922968d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1627964Reviewed-by: Richard Knoll <knollr@chromium.org>
Reviewed-by: Tim Song <tengs@chromium.org>
Auto-Submit: Vladislav Kaznacheev <kaznacheev@chromium.org>
Commit-Queue: Tim Song <tengs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#663096}

ui/message_center/views/message_popup_view.cc

@@ -113,11 +113,11 @@ void MessagePopupView::AutoCollapse() {
 
 void MessagePopupView::Show() {
   views::Widget::InitParams params(views::Widget::InitParams::TYPE_POPUP);
+  params.keep_on_top = true;
+#if defined(OS_LINUX) && !defined(OS_CHROMEOS)
   // Make the widget explicitly activatable as TYPE_POPUP is not activatable by
   // default but we need focus for the inline reply textarea.
   params.activatable = views::Widget::InitParams::ACTIVATABLE_YES;
-  params.keep_on_top = true;
-#if defined(OS_LINUX) && !defined(OS_CHROMEOS)
   params.opacity = views::Widget::InitParams::OPAQUE_WINDOW;
 #else
   params.opacity = views::Widget::InitParams::TRANSLUCENT_WINDOW;