[a11y]: fixes regression in image descriptions

In
https://crrev.com/c/5018206
we migrated to ScopedAccessibilityMode owned by AccessibilityLabelsService.

This replaced
ChromeContentBrowserClient::GetAXModeForBrowserContext
which had been called within
WebContentsImpl::WebContentsImpl.

The issue is on startup,
AccessibilityLabelsService::OnImageLabelsEnabledChanged
wouldn't be called (since prefs were not actually changed).

This leads to the common case for users who have the pref set, re-launch Chrome, and get no image descriptions.

R=grt@chromium.org

Fixed: 332733041
Test: new test; fails without the production change.
Change-Id: I49cb6bc1511616a1f6f5ff8bedc604af79f1b559
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5427622


Reviewed-by: Kyungjun Lee <kyungjunlee@google.com>
Commit-Queue: David Tseng <dtseng@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1283275}