[Autofill Assistant] Fix bug in ShowProgressBarAction.

Before this patch, when ShowProgressBarAction had no progress set or
progress set to 0, the action would return without calling the callback.
In this case, the script would just hang. This was broken in
http://crrev/c/1445902

With this patch, scripts with 0 progress continue without changing the
progress in any ways. This is guaranteed by Controller::SetProgress,
which only takes progress increases into account. Also, with this patch,
the message of ShowProgressBarAction is taken into account, even if the
progress is unset or set to 0.

Bug: 806868
Change-Id: Id48e43a2e418bfdf307596f93323ff8c873699eb
Reviewed-on: https://chromium-review.googlesource.com/c/1460922Reviewed-by: Clemens Arbesser <arbesser@google.com>
Commit-Queue: Stephane Zermatten <szermatt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#630319}

components/autofill_assistant/browser/actions/show_progress_bar_action.cc

@@ -24,11 +24,6 @@ ShowProgressBarAction::~ShowProgressBarAction() {}
 void ShowProgressBarAction::InternalProcessAction(
     ActionDelegate* delegate,
     ProcessActionCallback callback) {
-  if (proto_.show_progress_bar().progress() == 0) {
-    // Old script might still contain a ShowProgressBar action that clears the
-    // progress. Ignore these.
-    return;
-  }
   if (!proto_.show_progress_bar().message().empty()) {
     delegate->SetStatusMessage(proto_.show_progress_bar().message());
   }