MacPWAs: Ensure correct RenderWidgetHostViewCocoa ordering

A WebContentsViewCocoa can have several RenderWidgetHostViewCocoas
added as subviews. Each view, when added as a subview, in the function
WebContentsViewMac::CreateViewForWidget, is put behind all other
subviews (in contrast to the default behavior of -[NSView addSubview],
which puts the new subview in front).

The analogous code in the app shim, in the function
RenderWidgetHostNSViewBridge::SetParentWebContentsNSView, orders the
subviews in the default way, which does not match the behavior of the
browser.

As a result, the NSView ordering can get messed up, resulting in
un-clickable contents. This is vaguely similar to the issue in
crbug.com/859834, which also involve the login page. The login page
hits these sorts of bugs because RenderWidgetHostViewMacs are added
to a single WebContentsViewMac at the login page.

Bug: 1017446
Change-Id: I11425603b258b47e84c6e3df55ec966cf5246191
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1892577Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: ccameron <ccameron@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711237}

content/app_shim_remote_cocoa/render_widget_host_ns_view_bridge.mm

@@ -72,7 +72,12 @@ void RenderWidgetHostNSViewBridge::SetParentWebContentsNSView(
   // done by WebContentsViewMac.
   [cocoa_view_ setFrame:[parent_ns_view bounds]];
   [cocoa_view_ setAutoresizingMask:NSViewWidthSizable | NSViewHeightSizable];
-  [parent_ns_view addSubview:cocoa_view_];
+  // Place the new view below all other views, matching the behavior in
+  // WebContentsViewMac::CreateViewForWidget.
+  // https://crbug.com/1017446
+  [parent_ns_view addSubview:cocoa_view_
+                  positioned:NSWindowBelow
+                  relativeTo:nil];
 }
 
 void RenderWidgetHostNSViewBridge::MakeFirstResponder() {