Remove translation-related NotificationService notifications from tests

NOTIFICATION_TAB_LANGUAGE_DETERMINED and NOTIFICATION_PAGE_TRANSLATED
waiters are removed in favor of a helper class that observes the tab's
ContentTranslateDriver.

Also, ChromeTranslateClient::translate_driver() is changed to return
a pointer instead of a non-const ref.

Bug: 268984
Change-Id: I450ace7f647cf9418a28ac36aef70bf1f97a3f17
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1724420
Reviewed-by: anthonyvd <[email protected]>
Reviewed-by: Avi Drissman <[email protected]>
Commit-Queue: Evan Stade <[email protected]>
Cr-Commit-Position: refs/heads/master@{#683724}
16 files changed