Properly detect line height of Quick Draft textarea.

Merges [27021] to the 3.8 branch. Half was committed in [27013].

props batmoo.
fixes #26915.

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3.8 branch: Undo the colors.css changes in [26938] and move the rules to wp-admin.css.

This will prevent us from shipping a full re-build of the color schemes in 3.8.1.

see #26699.

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