Update screen.orientation to follow the latest version of the specs.

Source/modules/screen_orientation/ScreenOrientation.cpp

Index: Source/modules/screen_orientation/ScreenOrientation.cpp
diff --git a/Source/modules/screen_orientation/ScreenOrientation.cpp b/Source/modules/screen_orientation/ScreenOrientation.cpp
index c3599c158db9445aea71d29f3973b8deb0c35ca1..6bf1ba750c008f061f4a73ba971e859456cd3409 100644
--- a/Source/modules/screen_orientation/ScreenOrientation.cpp
+++ b/Source/modules/screen_orientation/ScreenOrientation.cpp
@@ -124,8 +124,10 @@ ScreenOrientation::~ScreenOrientation()
 const AtomicString& ScreenOrientation::orientation(Screen& screen)
 {
     ScreenOrientation& screenOrientation = ScreenOrientation::from(screen);
-    if (!screenOrientation.document())
-        return emptyAtom;
+    if (!screenOrientation.document()) {
+        // FIXME: we should try to return a better guess, like the latest known value.
+        return orientationToString(blink::WebScreenOrientationPortraitPrimary);
+    }
     ScreenOrientationController& controller = ScreenOrientationController::from(*screenOrientation.document());
     return orientationToString(controller.orientation());
 }