[Autofill Assistant] Only show overlay while running scripts.

Before this patch, the overlay was displayed when Autofill Assistant was
created.

This is confusing: the overlay is meant to reflect the fact that scripts
are running, but no scripts are running at that point and there might
never be any scripts.

With this patch, the overlay is only displayed when a script is run.

Bug: 806868
Change-Id: I2e1916a0253efb164b4b000c92f2cfef6a1703ad
Reviewed-on: https://chromium-review.googlesource.com/c/1264677Reviewed-by: Ganggui Tang <gogerald@chromium.org>
Commit-Queue: Ganggui Tang <gogerald@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597154}

components/autofill_assistant/browser/controller.cc

@@ -104,7 +104,6 @@ Controller::Controller(
   }
 
   GetUiController()->SetUiDelegate(this);
-  GetUiController()->ShowOverlay();
   if (!web_contents->IsLoading()) {
     GetOrCheckScripts(web_contents->GetLastCommittedURL());
   }