Number of frames in the current video content. (Read Only)
请注意，此值可能会随着 frameCount 在播放期间的变化而有所调整。
对于 URL 源，只有在源准备完成后才会设置。请参阅 VideoPlayer.Prepare。
Note: On WebGL, because the frame rate is not known, the frame count assumes a rate of 24FPS. See VideoPlayer.frameRate.