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The separation into different helpers was obscuring a lot of code duplication.
This de-dupes things and will make it easier to fix some bugs in tab
insertion/closing.

Bug: none
Change-Id: I8b26a1f973f8b2ce6c4bebcc0168f5c2dfcfb880
Reviewed-on: https://chromium-review.googlesource.com/1121789Reviewed-by: Allen Bauer <kylixrd@chromium.org>
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572000}