We are seeing a cluster of crashes that could be due to using an EGLSurface whose ANativeWindow has become invalid. This could happen if we continued to use (i.e. draw with) an EGLSurface after SurfaceHolder::onSurfaceDestroyed() has returned. This new flag enables an assertion that the native window is valid at the time of makeCurrent(), which happens early in the frame. BUG=[330392256]