[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.cc b/chrome/browser/flag_descriptions.cc
index a67ddd3..493c5f2e 100644
--- a/chrome/browser/flag_descriptions.cc
+++ b/chrome/browser/flag_descriptions.cc
@@ -2560,6 +2560,11 @@
 const char kAutofillAssistantDirectActionsDescription[] =
     "When enabled, expose direct actions from the Autofill Assistant.";
 
+const char kAutofillAssistantProactiveHelpName[] =
+    "Autofill Assistant proactive help";
+const char kAutofillAssistantProactiveHelpDescription[] =
+    "When enabled, allows the Autofill Assistant to proactively trigger.";
+
 const char kAutofillUseMobileLabelDisambiguationName[] =
     "Autofill Uses Mobile Label Disambiguation";
 const char kAutofillUseMobileLabelDisambiguationDescription[] =