[Extensions] Use BrowserWindowInterface in ExtensionContextMenuModel

This CL refactors the main constructor of ExtensionContextMenuModel to
use BrowserWindowInterface instead of Browser. This change is part of a
larger effort to decouple extensions from the Browser object.

For the few instances that are not yet supported by
BrowserWindowInterface, this CL uses
browser->GetBrowserForMigrationOnly() and enables it only on
non-Android platforms.

The Android constructor is not updated because it is a placeholder
being used by ExtensionActionContextMenuBridge while the shared
extensions logic is being refactored.

This change also required converting the context menu model unit test
to a browser test. This was done by:
  - Using ExtensionBrowserTest instead of ExtensionServiceTestBase
  - Using NavigateToURLInNewTab() instead of adding a URL using
    content::WebContentsTester. URLs must be provided via
    embedded_test_server, which uses http.

The test is only enabled for non-Android since it only tests the "main"
constructor.

Bypass-Check-License: converted unit test to browser test
Bug: 437774758
Change-Id: I6e5645fde57d693a6813c8ee82f4dabb1dceda43
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6966931
Commit-Queue: Emilia Paz <emiliapaz@chromium.org>
Reviewed-by: Devlin Cronin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1521427}