[Autofill] Merge `believe_server` into PreferHeuristicOverServer()

Before this CL, we have:

  bool PreferHeuristicOverServer(...) {
    return Foo();
  }

  if (!PreferHeuristicOverServer(...)) {
    bool believe_server = ...;
    if (believe_server) {
      Bar();
    }
  }

After this CL, we have:

  bool PreferHeuristicOverServer(...) {
    if (Foo()) {
      return true;
    }
    bool believe_server = ...;
    return !believe_server;
  }

  if (!PreferHeuristicOverServer(...)) {
    Bar();
  }

This CL is a no-op: The old and new code both call Bar() iff
  !Foo() && believe_server.

Two followup CLs simplify the resulting code.

Bug: 40100455
Change-Id: If6b33de66ecd134de7ac3da825bf10459f08b6d2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6830403
Reviewed-by: Dominic Battré <battre@chromium.org>
Commit-Queue: Christoph Schwering <schwering@google.com>
Cr-Commit-Position: refs/heads/main@{#1498672}