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This is a reland of 426df2be

Original change's description:
> Increase coverage for EULA browser tests
>
> Adds tests for:
> * Clicking on "System security settings" button in EULA screen
> * Accepting and rejecting usage stats collection
> * Clicking on "Learn more" button in EULA screen
>
> Removes dead code from EulaScreen class.
>
> Adds a HelpApp extension manifest to use in tests and modifies
> HelpAppLauncher class to allow tests to set the extension id.
>
> Adds a HelpAppTestHelper class to facilitate writing tests that
> open HelpApp dialogs.
>
> Change-Id: Id7841be5bf3faabb067d3a75d3b034bdf0e00e32
> Reviewed-on:
https://chromium-review.googlesource.com/c/chromium/src/+/1554123
> Commit-Queue: Tony De Luna <tonydeluna@chromium.org>
> Reviewed-by: Ryo Hashimoto <hashimoto@chromium.org>
> Reviewed-by: Toni Baržić <tbarzic@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#648759}

TBR=hashimoto

Bug: 947358
Change-Id: I23865c1957f300f55812f3275e830772aa27c32d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1560146Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Commit-Queue: Tony De Luna <tonydeluna@chromium.org>
Cr-Commit-Position: refs/heads/master@{#649258}