This patch adds the following messages to the autocomplete item:
1. For address and credit card autofill, use the `FormAutofill:FillForm` message.
2. For login autofill, use the `PasswordManager:OnFieldAutoComplete` message.
3. For generated password autofill, use the `PasswordManager:FillGeneratedPassword` message.
After adding the above three messages, every autocomplete item that performs an action
upon being clicked has a corresponding `fillMessageName`.
Here is how the new architecture works:
1. Whenever a user selects an autocomplete entry, the `AutoCompleteChild` sends
the `AutoComplete:SelectEntry` message to the parent process.
2. `AutoCompleteParent` extracts `fillMessageName` from the message and uses
the prefix of the message name to determine which actor should process this message.
3. The `OnFieldAutoCompleteEntrySelected` method of the actor is called. The actor in
the parent process determines what action to take.
Differential Revision: https://phabricator.services.mozilla.com/D209353
