Add "contain: paint" to XR DOM Overlay UA style

As suggested in https://github.com/immersive-web/dom-overlays/issues/20#issuecomment-596816355

This ensures that top layer elements act as a containing block for
"position: fixed" descendants.

Bug: 1060731
Change-Id: Id5bedac6ecde75a11d83442c025c0bd1da4e1cc2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2098178Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Klaus Weidner <klausw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#749458}

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

@@ -2,6 +2,9 @@
     /* force a transparent background */
     background: rgba(0,0,0,0) !important;
 
+    /* act as containing block for descendants */
+    contain: paint !important;
+
     /* the following styling is identical to :fullscreen */
     position: fixed !important;
     top: 0 !important;