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This makes the Extensions menu use a standard title and close-x. The
footer entry is easier to notice and understand.

Removes work that's been done for custom headers. First-iteration work
is not looking to introduce panes to this dialog.

Bug: chromium:943702
Change-Id: Ic42fa05953be92f0a53448dc1a7f1c628a511dca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1566619
Commit-Queue: Peter Boström <pbos@chromium.org>
Commit-Queue: Taylor Bergquist <tbergquist@chromium.org>
Reviewed-by: Taylor Bergquist <tbergquist@chromium.org>
Cr-Commit-Position: refs/heads/master@{#650519}