[Autofill Assistant] WebContent delegate handling.

Before this patch, we cleared the delegate for web_content
unconditionally. That means we would remove an already existing delegate
from web_content.

This patch introduces a bool in the controller to track whether the delegate
should be cleared in the destructor.

After this patch, we will no longer stomp on other components setting
a delegate in web_contents.

R=gogerald@chromium.org

Bug: 806868
Change-Id: I92ce1ef2dcf4a48f6709bddee0f76871a58c2df4
Reviewed-on: https://chromium-review.googlesource.com/c/1263118Reviewed-by: Ganggui Tang <gogerald@chromium.org>
Commit-Queue: Ganggui Tang <gogerald@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596978}

components/autofill_assistant/browser/controller.cc

@@ -89,6 +89,7 @@ Controller::Controller(
       allow_autostart_(true),
       periodic_script_check_scheduled_(false),
       periodic_script_check_count_(false),
+      clear_web_contents_delegate_(false),
       weak_ptr_factory_(this) {
   DCHECK(parameters_);
 
@@ -98,6 +99,7 @@ Controller::Controller(
   // of using the controller as a web_contents delegate. It may interfere with
   // an already existing delegate.
   if (web_contents->GetDelegate() == nullptr) {
+    clear_web_contents_delegate_ = true;
     web_contents->SetDelegate(this);
   }
 
@@ -109,7 +111,9 @@ Controller::Controller(
 }
 
 Controller::~Controller() {
-  web_contents()->SetDelegate(nullptr);
+  if (clear_web_contents_delegate_) {
+    web_contents()->SetDelegate(nullptr);
+  }
 }
 
 void Controller::GetOrCheckScripts(const GURL& url) {

components/autofill_assistant/browser/controller.h

@@ -114,6 +114,10 @@ class Controller : public ScriptExecutorDelegate,
   // Number of remaining periodic checks.
   int periodic_script_check_count_;
 
+  // Whether to clear the web_contents delegate when the controller is
+  // destroyed.
+  bool clear_web_contents_delegate_;
+
   base::WeakPtrFactory<Controller> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(Controller);