[Picture-in-Picture] Fix shadow dom crash when exiting PiP.

This CL fixes a tab crash that occurs when calling document.exitPictureInPicture()
if a video in a shadow DOM is already in Picture-in-Picture.

Bug: 904828

Change-Id: I6aed0fa43dd0af1406d4349509101d423f622d96
Reviewed-on: https://chromium-review.googlesource.com/c/1333769Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Commit-Queue: François Beaufort <beaufort.francois@gmail.com>
Cr-Commit-Position: refs/heads/master@{#607658}

third_party/WebKit/LayoutTests/external/wpt/picture-in-picture/shadow-dom.html

@@ -58,5 +58,6 @@ promise_test(async t => {
     assert_equals(ids.root4.pictureInPictureElement, null);
     assert_equals(ids.root5.pictureInPictureElement, null);
   })
+  .then(() => document.exitPictureInPicture());
 });
 </script>

third_party/blink/renderer/modules/picture_in_picture/document_picture_in_picture.cc

@@ -32,8 +32,7 @@ ScriptPromise DocumentPictureInPicture::exitPictureInPicture(
     Document& document) {
   PictureInPictureControllerImpl& controller =
       PictureInPictureControllerImpl::From(document);
-  Element* picture_in_picture_element =
-      controller.PictureInPictureElement(document);
+  Element* picture_in_picture_element = controller.PictureInPictureElement();
 
   if (!picture_in_picture_element) {
     return ScriptPromise::RejectWithDOMException(

third_party/blink/renderer/modules/picture_in_picture/picture_in_picture_controller_impl.cc

@@ -193,6 +193,10 @@ void PictureInPictureControllerImpl::OnPictureInPictureControlClicked(
   }
 }
 
+Element* PictureInPictureControllerImpl::PictureInPictureElement() const {
+  return picture_in_picture_element_;
+}
+
 Element* PictureInPictureControllerImpl::PictureInPictureElement(
     TreeScope& scope) const {
   if (!picture_in_picture_element_)

third_party/blink/renderer/modules/picture_in_picture/picture_in_picture_controller_impl.h

@@ -46,6 +46,7 @@ class PictureInPictureControllerImpl : public PictureInPictureController {
   Status IsDocumentAllowed() const;
 
   // Returns element currently in Picture-in-Picture if any. Null otherwise.
+  Element* PictureInPictureElement() const;
   Element* PictureInPictureElement(TreeScope&) const;
 
   // Implementation of PictureInPictureController.