wpviews: avoid resizing iframes after undo/redo

[iseulde]

We could save the height from the first view in the instance so we can reuse it later.
This will prevent the editor content from moving around when undoing or redoing changes.

[iseulde]

In 32711:

TinyMCE: wpviews: cache iframe heights

Cache iframe heights per instance so it can be reused. This will prevent to content from moving in the editor when undoing or redoing changes.

Fixes #32593.