Added maxwalker, for an opinion on the layout I suggest. 

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LGTM! We have a max-width for the content (#body-container) so we won't get an overly wide text field on bigger screens. For narrow widths (below 480px) the text field would become full-width and the pull-down button would move below the link, correct?

Yes, that would be the intention.

Thanks for the input.

Shouldn't the button be *above* the text box?

The button is vertically aligned to the middle of the flag row. It's unintentional it aligns with the top of the textarea. It will depend on the length of the description.

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#3 implies that in a narrow width, we would have

  Description
  Text Box
  Link
  DropDown

This seems strange in that the textbox is logically subordinate to the drop-down. 

Why does it seem strange to you?

Showing the pull-down button below the dropbox seems better to me:
- It's a common pattern (for example in dialogs or forms) that the buttons at the bottom confirm the text input above.
- It keeps intact the visual order of items (title first, button last) as you scroll through the list of flags.

So the order would be:
1. Description
2. Text field
3. Link
4. Pull-down button

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The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/b3aa36a855f37cb190c2a3ad75d85bd0ead199a3

commit b3aa36a855f37cb190c2a3ad75d85bd0ead199a3
Author: Mustafa Emre Acer <meacer@chromium.org>
Date: Tue May 22 21:44:05 2018

Add support to chrome://flags for command line flags that need to be treated as a list of origins

This CL adds support for command line flags that are lists of url::Origins. E.g. The value in --flag=value will now be modifyable from the chrome://flags page using a free form textbox, and will be treated as a comma separated list of origins (e.g. http://example1.test,http://example2.test)

The string from the textbox is transformed as follows:
- Continuous whitespace characters are collapsed into single a space
- String is tokenized using space and comma as delimiters
- Each token is parsed as a GURL. Invalid URLs or URLs with a scheme other than http and https are discarded.
- Remaining URLs are converted to url::Origins, then joined into a single, comma separated string.

The CL also adds --unsafely-treat-insecure-origin-as-secure as the first such flag to chrome://flags. Developers will now be able to modify the list of insecure origins treated as secure from the chrome://flags page on all platforms.

Bug:  834381 
Change-Id: Iad44b5b2724687c7bea1ae45c23ccc910eb5cc9f
Reviewed-on: https://chromium-review.googlesource.com/1038152
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Mattias Nissler <mnissler@chromium.org>
Reviewed-by: Mike West <mkwst@chromium.org>
Reviewed-by: Edward Jung <edwardjung@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Mustafa Emre Acer <meacer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#560791}
[modify] https://crrev.com/b3aa36a855f37cb190c2a3ad75d85bd0ead199a3/chrome/browser/about_flags.cc
[modify] https://crrev.com/b3aa36a855f37cb190c2a3ad75d85bd0ead199a3/chrome/browser/about_flags.h
[add] https://crrev.com/b3aa36a855f37cb190c2a3ad75d85bd0ead199a3/chrome/browser/about_flags_browsertest.cc
[modify] https://crrev.com/b3aa36a855f37cb190c2a3ad75d85bd0ead199a3/chrome/browser/about_flags_unittest.cc
[modify] https://crrev.com/b3aa36a855f37cb190c2a3ad75d85bd0ead199a3/chrome/browser/flag_descriptions.cc
[modify] https://crrev.com/b3aa36a855f37cb190c2a3ad75d85bd0ead199a3/chrome/browser/flag_descriptions.h
[modify] https://crrev.com/b3aa36a855f37cb190c2a3ad75d85bd0ead199a3/chrome/browser/ui/webui/flags_ui.cc
[modify] https://crrev.com/b3aa36a855f37cb190c2a3ad75d85bd0ead199a3/chrome/test/BUILD.gn
[modify] https://crrev.com/b3aa36a855f37cb190c2a3ad75d85bd0ead199a3/components/flags_ui/BUILD.gn
[modify] https://crrev.com/b3aa36a855f37cb190c2a3ad75d85bd0ead199a3/components/flags_ui/DEPS
[modify] https://crrev.com/b3aa36a855f37cb190c2a3ad75d85bd0ead199a3/components/flags_ui/feature_entry.h
[modify] https://crrev.com/b3aa36a855f37cb190c2a3ad75d85bd0ead199a3/components/flags_ui/feature_entry_macros.h
[modify] https://crrev.com/b3aa36a855f37cb190c2a3ad75d85bd0ead199a3/components/flags_ui/flags_state.cc
[modify] https://crrev.com/b3aa36a855f37cb190c2a3ad75d85bd0ead199a3/components/flags_ui/flags_state.h
[modify] https://crrev.com/b3aa36a855f37cb190c2a3ad75d85bd0ead199a3/components/flags_ui/flags_state_unittest.cc
[modify] https://crrev.com/b3aa36a855f37cb190c2a3ad75d85bd0ead199a3/components/flags_ui/flags_ui_constants.cc
[modify] https://crrev.com/b3aa36a855f37cb190c2a3ad75d85bd0ead199a3/components/flags_ui/flags_ui_constants.h
[modify] https://crrev.com/b3aa36a855f37cb190c2a3ad75d85bd0ead199a3/components/flags_ui/resources/flags.css
[modify] https://crrev.com/b3aa36a855f37cb190c2a3ad75d85bd0ead199a3/components/flags_ui/resources/flags.html
[modify] https://crrev.com/b3aa36a855f37cb190c2a3ad75d85bd0ead199a3/components/flags_ui/resources/flags.js

This was implemented as part of chrome://flags page: There is now a text entry on chrome://flags where developers can enter a list of origins.