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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git-svn-id: http://core.svn.wordpress.org/branches/3.8@26898 1a063a9b-81f0-0310-95a4-ce76da25c4cd