[Autofill Assistant] Add proactive help to assistant settings

This change moves the Autofill Assistant settings into a separate
screen and adds a new toggle for proactive help.
The Autofill Assistant sub-section is now initially visible
to all users (as opposed to before where it would just appear
after the first use).

Other smaller changes:
 - the link to the settings in the assistant's bottomsheet now
   points to the new page
 - Rejecting the onboarding now leaves the preference unset instead
   of setting it to false.

Bug: b/168178344
Change-Id: I76f419a46598cddb003d95ac8be317c21de72a93
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2505875
Commit-Queue: Clemens Arbesser <[email protected]>
Reviewed-by: Boris Sazonov <[email protected]>
Cr-Commit-Position: refs/heads/master@{#825856}
diff --git a/chrome/browser/flag_descriptions.h b/chrome/browser/flag_descriptions.h
index 0977276..1356aea 100644
--- a/chrome/browser/flag_descriptions.h
+++ b/chrome/browser/flag_descriptions.h
@@ -1495,6 +1495,9 @@
 extern const char kAutofillAssistantDirectActionsName[];
 extern const char kAutofillAssistantDirectActionsDescription[];
 
+extern const char kAutofillAssistantProactiveHelpName[];
+extern const char kAutofillAssistantProactiveHelpDescription[];
+
 extern const char kAutofillUseMobileLabelDisambiguationName[];
 extern const char kAutofillUseMobileLabelDisambiguationDescription[];