Support subtitles for native fullscreen video

Source/web/FullscreenController.cpp

Index: Source/web/FullscreenController.cpp
diff --git a/Source/web/FullscreenController.cpp b/Source/web/FullscreenController.cpp
index 6268f88b85c9fa708c8a8b2820135670825827bc..ded893651ec0c1f2570416ad29b74bdc10a7fc81 100644
--- a/Source/web/FullscreenController.cpp
+++ b/Source/web/FullscreenController.cpp
@@ -83,6 +83,12 @@ void FullscreenController::didEnterFullScreen()
             }
 
             FullscreenElementStack::from(doc)->webkitDidEnterFullScreenForElement(0);
+#if USE(NATIVE_FULLSCREEN_VIDEO)
+            if (Element* element = FullscreenElementStack::currentFullScreenElementFrom(doc)) {
+                if (element->isMediaElement() && m_webViewImpl->layerTreeView())
+                    m_webViewImpl->layerTreeView()->setHasTransparentBackground(true);
+            }
+#endif
         }
     }
 }
@@ -103,6 +109,10 @@ void FullscreenController::willExitFullScreen()
             fullscreen->webkitCancelFullScreen();
             m_isCancelingFullScreen = false;
             fullscreen->webkitWillExitFullScreenForElement(0);
+#if USE(NATIVE_FULLSCREEN_VIDEO)
+            if (m_webViewImpl->layerTreeView())
+                m_webViewImpl->layerTreeView()->setHasTransparentBackground(m_webViewImpl->isTransparent());
+#endif
         }
     }
 }