Skip requestFullscreen checks on current fullscreen element

This will allow an already fullscreen element to switch displays
(with window-placement permission) without requiring an additional
user activation.  This is similar to how moveTo behaves.

Filed here: whatwg/fullscreen#193

Bug: 1218483
Change-Id: Ia793eccb7e3f25492690c703025f7e975b597ccf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2970683
Reviewed-by: Mustaq Ahmed <[email protected]>
Commit-Queue: enne <[email protected]>
Cr-Commit-Position: refs/heads/master@{#897516}
NOKEYCHECK=True
GitOrigin-RevId: c17358d64ea9ee12cce4d136ac41dc7b1b90421c

blink/renderer/core/fullscreen/fullscreen.cc

@@ -277,7 +277,7 @@ bool AllowedToUseFullscreen(const Document& document,
       mojom::blink::PermissionsPolicyFeature::kFullscreen, report_on_failure);
 }
 
-bool AllowedToRequestFullscreen(Document& document) {
+bool AllowedToRequestFullscreen(Document& document, Element& element) {
   //  WebXR DOM Overlay integration, cf.
   //  https://immersive-web.github.io/dom-overlays/
   //
@@ -308,6 +308,12 @@ bool AllowedToRequestFullscreen(Document& document) {
     return false;
   }
 
+  // If the element is already fullscreen, then it is allowed to repeat a
+  // request to fullscreen (possibly on another display) without requiring
+  // user activation.
+  if (element == Fullscreen::FullscreenElementFrom(document))
+    return true;
+
   // An algorithm is allowed to request fullscreen if one of the following is
   // true:
 
@@ -390,7 +396,7 @@ bool RequestFullscreenConditionsMet(Element& pending, Document& document) {
     return false;
 
   // This algorithm is allowed to request fullscreen.
-  if (!AllowedToRequestFullscreen(document))
+  if (!AllowedToRequestFullscreen(document, pending))
     return false;
 
   return true;