Fix incorrect widget area layout for new unified dashboard content area.

[NaotoNakamura]

Summary

Addresses issue #4268

Relevant technical choices

Fix incorrect widget area layout for new unified dashboard content area

Checklist

• My code is tested and passes existing unit tests.
• My code has an appropriate set of unit tests which all pass.
• My code is backward-compatible with WordPress 4.7 and PHP 5.6.
• My code follows the WordPress coding standards.
• My code has proper inline documentation.
• I have added a QA Brief on the issue linked above.
• I have signed the Contributor License Agreement (see https://cla.developers.google.com/).

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[NaotoNakamura]

@googlebot
I signed it!

[felixarntz]

Thanks @NaotoNakamura, looks great!