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Adds a header area to the top of the Extensions menu. This is not done
as a bubble title since the area needs to have non-standard buttons in
the captions area + transitions will be made between different pages.

Bug: chromium:943702
Change-Id: Ie9c62e0f1d9ea5cf5e53b94a18c5810ca83112af
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1548244
Commit-Queue: Peter Boström <pbos@chromium.org>
Reviewed-by: Taylor Bergquist <tbergquist@chromium.org>
Cr-Commit-Position: refs/heads/master@{#646619}