Simplify construction mechanism of AutofillProvider.

Prior to this CL, the base class, AutofillProvider, attached itself
to the WebContents in its constructor. This meant that derived classes
would have to be instantiated using new and that the resulting pointer
was already owned elsewhere.

This changes mirrors the setup of, e.g., ContentAutofillClient and
ChromeAutofillClient. While the parent class is WebContentsUserData
(which is convenient for embedders that only deal with the base class),
it does not pass ownership of itself to the WebContents. That part is
left to the classes extending it.

This way, the following pattern works:
// Creation:
AutofillProviderAndroid::CreateForWebContents(contents);

// Use elsewhere:
AutofillProvider::FromWebContents(contents)->DoSomething()

Bug: 1478934
Change-Id: I037f342fa1c04cca4832d08c1a7c97b98ffaa07a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4863562
Commit-Queue: Jan Keitel <jkeitel@google.com>
Reviewed-by: Dominic Battre <battre@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1196566}