Text in a `2021-foreground` `amp-story-grid-layer` should not resize on vertical viewport change

### Description

When adding text in a `2021-foreground` preset `amp-story-grid-layer`, the size grows as the viewport grows, while the grid layer itself doesn't. The layer's text size should be defined as a function of the layer height, not the viewport height.

This video makes it a bit clearer:

https://user-images.githubusercontent.com/4594196/136186959-b37e688d-b55a-4bc0-a47a-fdf209ee0bd7.mov

### Reproduction Steps

Add text to an `amp-story-grid-layer` with the `2021-foreground` preset, and resize the viewport to a size taller than the `69 / 116` layer aspect ratio. The text starts growing in size, but it shouldn't.

### Relevant Logs

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### Browser(s) Affected

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### OS(s) Affected

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### Device(s) Affected

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### AMP Version Affected

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Start by reproducing the behavior with an amp-story-grid-layer using the 2021-foreground preset, following the viewport resize steps in the issue. Trace the implementation that sizes text relative to the layer and viewport; done means the text size remains tied to the layer height when the vertical viewport grows beyond the 69/116 aspect ratio.

Written by the indexing model from the issue text.