File Editor: Eliminate gap inserted between code editor and heading in IE11...

File Editor: Eliminate gap inserted between code editor and heading in IE11 when CodeMirror initializes.

Props afercia, melchoyce, Clorith for testing, SergeyBiryukov for testing.
See #12423.
Fixes #42554 for trunk.

Built from https://develop.svn.wordpress.org/trunk@42189


git-svn-id: https://core.svn.wordpress.org/trunk@42019 1a063a9b-81f0-0310-95a4-ce76da25c4cd