Reland "[Office upload] Use BrowserListObserver to wait for created Files app"

This is a reland of commit 9089c92681121f77bda5c622d902dd94cc0aa80d

This is a clean reland. The failure was caused by a flaky test,
fixed in crrev.com/c/4463978, crrev.com/c/4478591

Original change's description:
> [Office upload] Use BrowserListObserver to wait for created Files app
>
> When we didn't have a Files app window to use as a modal parent for
> our dialog, we created a new Files app window and used it as the
> parent. But this didn't work because the callback we provide to
> ShowItemInFolder gets called early in the launch process, before
> the actual Files app is launched.
>
> Instead use BrowserListObserver to catch when a new Browser is
> added, check that it's the Files app window we were expecting,
> and launch our dialog with this window as the modal parent.
>
> Several tests didn't use "real" files, they just created
> FileSystemURLs to plausible locations. This meant that
> ShowItemInFolder() failed to launch Files app in tests, which
> now causes CloudOpenTask to never make progress. Use the default
> browser() in several tests to avoid this problem.
>
> Update NewModalParentCreatedWithExisting to more directly
> capture and check the Browser that was created by CloudOpenTask.
>
> Bug: b:275950760
> Change-Id: I4f3b9f1891e0b32a2c426333eebcd5c742359833
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4431739
> Commit-Queue: Peter Marshall <petermarshall@chromium.org>
> Reviewed-by: Cassy Chun-Crogan <cassycc@google.com>
> Code-Coverage: Findit <findit-for-me@appspot.gserviceaccount.com>
> Cr-Commit-Position: refs/heads/main@{#1133586}

Cq-Include-Trybots: luci.chrome.try:linux-chromeos-chrome
Bug: b:275950760
Change-Id: I00ad795ce62ad7e21b37507600ea42ae7b98ae61
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4464338
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Cassy Chun-Crogan <cassycc@google.com>
Code-Coverage: Findit <findit-for-me@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#1135772}

• chrome/browser/ash/file_manager/file_tasks_browsertest.cc[diff]
• chrome/browser/ui/webui/ash/cloud_upload/cloud_upload_dialog.cc[diff]
• chrome/browser/ui/webui/ash/cloud_upload/cloud_upload_dialog.h[diff]
• chrome/browser/ui/webui/ash/cloud_upload/cloud_upload_dialog_browsertest.cc[diff]