[EVP] Show a toast when emails are verified in autofill.

In this CL, we make the EmailVerifierDelegate call a callback whenever
an email gets successufllyl verified, so that the callback can display
a toast to notify the user.

The callback is implemented by the ChromeAutofillClient, which directs
the event to the toast service.

UX: https://docs.google.com/presentation/d/16I0l_TeHMr2ivwkV3_Tl25GnrpKcp5mLYuyokud8PW0/edit?resourcekey=0-OK-6BoAXDDnC7KJnZHerqA&slide=id.g3616e47c1f3_0_0&pli=1&authuser=0#slide=id.g3616e47c1f3_0_0
Bug: 380367784
Change-Id: I5df8291d1f10723988a70d657d0e926d60a0898b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6990669
Commit-Queue: Sam Goto <goto@chromium.org>
Reviewed-by: Christoph Schwering <schwering@google.com>
Cr-Commit-Position: refs/heads/main@{#1522675}