Remove isWindowToken check from requestScrollCapture

The 'behindClient' argument is optional, it is used to filter
window selection to those below the give window (the UI of the
screenshots tool itself).

Change-Id: I33696c79dfbbf7d06410384745ce75ccce837594

services/core/java/com/android/server/wm/WindowManagerService.java

@@ -6925,9 +6925,6 @@ public class WindowManagerService extends IWindowManager.Stub
         if (!checkCallingPermission(READ_FRAME_BUFFER, "requestScrollCapture()")) {
             throw new SecurityException("Requires READ_FRAME_BUFFER permission");
         }
-        if (behindClient != null && !isWindowToken(behindClient)) {
-            throw new IllegalArgumentException("behindClient must be a window token");
-        }
         final long token = Binder.clearCallingIdentity();
         try {
             synchronized (mGlobalLock) {