[Mahi] Use MahiManagerImpl in the Mahi UI browser tests

This CL makes the Mahi UI browser tests in a more end-to-end fashion
by using MahiManagerImpl instead of a fake Mahi UI manager.

To achieve this, this CL:

1. Creates a web page to trigger the Mahi functionality.

2. Uses a fake Mahi provider during testing.

This CL helps to increase test coverage on MahiManagerImpl, making
the subsequent changes on MahiManagerImpl easier to verify.

This CL should not bring any noticeable product changes.

Bug: b:348042178,b:319735347
Change-Id: I1f217b1c388b46105bfb37e615a263474e501b55
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5685533
Reviewed-by: Thanh Nguyen <thanhdng@chromium.org>
Reviewed-by: Xinglong Luan <alanlxl@chromium.org>
Reviewed-by: Andre Le <leandre@chromium.org>
Commit-Queue: Andrew Xu <andrewxu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1325259}

• chrome/browser/ash/mahi/BUILD.gn[diff]
• chrome/browser/ash/mahi/mahi_manager_impl.cc[diff]
• chrome/browser/ash/mahi/mahi_manager_impl.h[diff]
• chrome/browser/ash/mahi/mahi_ui_browsertest.cc[diff]
• chrome/test/data/mahi/test_article.html[Added - diff]
• components/manta/mahi_provider.h[diff]
• components/manta/manta_service.h[diff]