Reland "Reland "Increase coverage for EULA browser tests""

This is a reland of c3e3793b3e7b8ac42b35aea22ce28f69208132c1

Fixed use after free error in the previous patch detected by asan
tests.

Root cause was that the observer in HelpAppTestHelper::Waiter received
several updates about window visibility. On every update,  a copy of
the window pointer was added to the internal list of ScopedObserver.

After deleting the window, the pointers in the list of ScopedObserver
pointed to released memory and ScopedObserver's destructor tried to
reference these freed pointers.

Fixed by replacing ScopedObserver with a set of dialog window pointers.

Original change's description:
> This is a reland of 426df2bedfa0c4c6bf9bcccaef6caf981c9a426c
>
> 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/+/1560146
> Reviewed-by: Alexander Alekseev <alemate@chromium.org>
> Commit-Queue: Tony De Luna <tonydeluna@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#649258}

TBR=hashimoto
Bug: 947358

Change-Id: I3dfc8001d0c0a68ca354d57e7e7b9eed90cbb2c1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1562611
Commit-Queue: Tony De Luna <tonydeluna@chromium.org>
Reviewed-by: Toni Baržić <tbarzic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#650088}