Strengthen frame readiness while testing fromPixels with video element

[hujiajie]

This fixes the error in WebGPU-backed unit tests:

OperationError: Failed to execute 'importExternalTexture' on 'GPUDevice': Failed to import texture from video element that doesn't have back resource.

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This change is 

[hujiajie]

replaceHTMLVideoElementSource() in tfjs-data is exempted at the moment because it cannot be built on top of the proposed utils here. It might deserve a separate PR to deal with that.

[gyagp]

Can we further simplify the usage, like "await createAndWaitForVideo(src)"? Then you may hide all the details behind this function. Also for simplicity, we may not need to support multiple sources.

[hujiajie]

The unit test removes the video element when it's finished, so for symmetry document.body.appendChild(video) also lives at the top level. Meanwhile, it reads the autoplay policy may cause video.play() to fail if the element is not yet on screen. I feel it inevitably breaks createAndWaitForVideo() apart.

[gyagp]

According to browserstack tests, I think it's safe to use a single function. But anyway, it's already a good improvement comparing with exsiting code, so LGTM.

[pyu10055]

Reviewed 3 of 3 files at r1, all commit messages.
Reviewable status: complete! 2 of 1 approvals obtained (waiting on @lina128)

[pyu10055]

thanks!

Reviewable status: complete! 2 of 1 approvals obtained (waiting on @lina128)