WebXR: handle DOM overlay from OOPIF fullscreen

If entering WebXR mode using DOM Overlay when we're already in
fullscreen mode, and if we have a remote ancestor frame that's
in another process, the ancestor frame didn't get set to transparent
since fullscreen_controller ignores changes to fullscreen options.

As a workaround, exit fullscreen mode in this case before proceeding
with the session request.

Bug: 1101193
Change-Id: I36b9918717abf6c5d462b7c78d545ae53fc5391e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2358493Reviewed-by: Piotr Bialecki <bialpio@chromium.org>
Commit-Queue: Klaus Weidner <klausw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#799401}

third_party/blink/renderer/modules/xr/xr_system.cc

@@ -996,6 +996,37 @@ void XRSystem::RequestImmersiveSession(LocalFrame* frame,
   outstanding_request_queries_.insert(query);
   auto session_options = XRSessionOptionsFromQuery(*query);
 
+  // If using DOM Overlay, if we're already in fullscreen mode, and if this
+  // frame has a remote ancestor (OOPIF with the ancestor in another process),
+  // we need to exit and re-enter fullscreen mode to properly apply the
+  // is_xr_overlay property. Request a fullscreen exit, and continue with
+  // the session request once that completes.
+  if (query->DOMOverlayElement() && Fullscreen::FullscreenElementFrom(*doc)) {
+    bool has_remote_ancestor = false;
+    for (Frame* f = GetFrame(); f; f = f->Tree().Parent()) {
+      if (f->IsRemoteFrame()) {
+        has_remote_ancestor = true;
+        break;
+      }
+    }
+    DVLOG(2) << __func__ << ": has_remote_ancestor=" << has_remote_ancestor;
+    if (has_remote_ancestor) {
+      fullscreen_exit_observer_ =
+          MakeGarbageCollected<OverlayFullscreenExitObserver>(this);
+
+      base::OnceClosure callback =
+          WTF::Bind(&XRSystem::DoRequestSession, WrapWeakPersistent(this),
+                    WrapPersistent(query), std::move(session_options));
+      fullscreen_exit_observer_->ExitFullscreen(doc, std::move(callback));
+      return;
+    }
+  }
+  DoRequestSession(std::move(query), std::move(session_options));
+}
+
+void XRSystem::DoRequestSession(
+    PendingRequestSessionQuery* query,
+    device::mojom::blink::XRSessionOptionsPtr session_options) {
   // In DOM overlay mode, there's an additional step before an immersive-ar
   // session can start, we need to enter fullscreen mode by setting the
   // appropriate element as fullscreen from the Renderer, then waiting for the

third_party/blink/renderer/modules/xr/xr_system.h

@@ -318,7 +318,7 @@ class XRSystem final : public EventTargetWithInlineData,
   };
 
   // Native event listener used when waiting for fullscreen mode to fully exit
-  // when ending an XR session.
+  // when starting or ending an XR session.
   class OverlayFullscreenExitObserver : public NativeEventListener {
    public:
     explicit OverlayFullscreenExitObserver(XRSystem* xr);
@@ -366,6 +366,9 @@ class XRSystem final : public EventTargetWithInlineData,
                             PendingRequestSessionQuery* query,
                             ExceptionState* exception_state);
 
+  void DoRequestSession(
+      PendingRequestSessionQuery* query,
+      device::mojom::blink::XRSessionOptionsPtr session_options);
   void OnRequestSessionSetupForDomOverlay(
       PendingRequestSessionQuery*,
       device::mojom::blink::RequestSessionResultPtr result);