[ios] Fix crash related to download and prerendering

When a WebState is removed from the WebStateList while a download is
pending/started/complete, it needs to inform the delegate so that it
can be stopped.

This requires remembering in DownloadManagerTabHelper whether the
delegate has been started, which happens from `DidCreateDownload()`
if the WebState is visible by that point or `WasShown()`.

(cherry picked from commit 794decd)

Fixed: 1317293
Change-Id: I72dedbf8c03107405edb6b1fc4abf4e150dee951
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3592976
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Harry Souders <harrysouders@google.com>
Commit-Queue: Harry Souders <harrysouders@google.com>
Cr-Original-Commit-Position: refs/heads/main@{#993753}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3594442
Reviewed-by: Krishna Govind <govind@chromium.org>
Commit-Queue: Krishna Govind <govind@chromium.org>
Owners-Override: Krishna Govind <govind@chromium.org>
Cr-Commit-Position: refs/branch-heads/5005@{#33}
Cr-Branched-From: 5b4d945-refs/heads/main@{#992738}

ios/chrome/browser/download/download_manager_tab_helper.h

@@ -65,6 +65,7 @@ class DownloadManagerTabHelper
   web::WebState* web_state_ = nullptr;
   __weak id<DownloadManagerTabHelperDelegate> delegate_ = nil;
   std::unique_ptr<web::DownloadTask> task_;
+  bool delegate_started_ = false;
 
   WEB_STATE_USER_DATA_KEY_DECL();
 };

ios/chrome/browser/download/download_manager_tab_helper.mm

@@ -53,17 +53,26 @@
 
 void DownloadManagerTabHelper::SetDelegate(
     id<DownloadManagerTabHelperDelegate> delegate) {
+  if (delegate == delegate_)
+    return;
+
+  if (delegate_ && task_ && delegate_started_)
+    [delegate_ downloadManagerTabHelper:this didHideDownload:task_.get()];
+
+  delegate_started_ = false;
   delegate_ = delegate;
 }
 
 void DownloadManagerTabHelper::WasShown(web::WebState* web_state) {
-  if (task_) {
+  if (task_ && delegate_) {
+    delegate_started_ = true;
     [delegate_ downloadManagerTabHelper:this didShowDownload:task_.get()];
   }
 }
 
 void DownloadManagerTabHelper::WasHidden(web::WebState* web_state) {
-  if (task_) {
+  if (task_ && delegate_) {
+    delegate_started_ = false;
     [delegate_ downloadManagerTabHelper:this didHideDownload:task_.get()];
   }
 }
@@ -104,9 +113,12 @@
   }
   task_ = std::move(task);
   task_->AddObserver(this);
-  [delegate_ downloadManagerTabHelper:this
-                    didCreateDownload:task_.get()
-                    webStateIsVisible:web_state_->IsVisible()];
+  if (web_state_->IsVisible() && delegate_) {
+    delegate_started_ = true;
+    [delegate_ downloadManagerTabHelper:this
+                      didCreateDownload:task_.get()
+                      webStateIsVisible:true];
+  }
 }
 
 WEB_STATE_USER_DATA_KEY_IMPL(DownloadManagerTabHelper)