WebXR DOM Overlay: remove backdrop CSS modification

This isn't technically needed since the DOM overlay element effectively
acts as the root element while active, so the backdrop doesn't get drawn.

See also https://github.com/immersive-web/dom-overlays/issues/15

Bug: 991747
Change-Id: I23862f2fbc8434813bb4248695ff37cb62cd48e4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2078803Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Klaus Weidner <klausw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745617}

third_party/blink/renderer/core/css/webxr_overlay.css

@@ -21,8 +21,3 @@
     /* intentionally not !important */
     object-fit: contain;
 }
-
-/* If there's a backdrop from fullscreen mode, set that transparent. */
-:xr-overlay:fullscreen::backdrop {
-  background: rgba(0,0,0,0) !important;
-}

third_party/blink/renderer/core/dom/document.cc

@@ -271,6 +271,7 @@
 #include "third_party/blink/renderer/core/page/spatial_navigation_controller.h"
 #include "third_party/blink/renderer/core/paint/compositing/paint_layer_compositor.h"
 #include "third_party/blink/renderer/core/paint/first_meaningful_paint_detector.h"
+#include "third_party/blink/renderer/core/paint/paint_layer.h"
 #include "third_party/blink/renderer/core/paint/paint_layer_scrollable_area.h"
 #include "third_party/blink/renderer/core/probe/core_probes.h"
 #include "third_party/blink/renderer/core/resize_observer/resize_observer_controller.h"
@@ -3037,6 +3038,10 @@ void Document::SetIsXrOverlay(bool val, Element* overlay_element) {
     overlay_element->PseudoStateChanged(CSSSelector::kPseudoXrOverlay);
   }
 
+  // The DOM overlay may change the effective root element. Need to update
+  // compositing inputs to avoid a mismatch in CompositingRequirementsUpdater.
+  GetLayoutView()->Layer()->SetNeedsCompositingInputsUpdate();
+
   // Ensure that the graphics layer tree gets fully rebuilt on changes,
   // similar to HTMLVideoElement::DidEnterFullscreen(). This may not be
   // strictly necessary if the compositing changes are based on visibility