Remove base::Unretained usage in PasswordProtectionRequest

This usage was safe, since if the PasswordProtectionRequestion was
destroyed, |url_loader_| would be destroyed as well and not call the
callback. But the usage should be either documented with that reasoning
or removed, and we already have WeakPtrs set up for the
PasswordProtectionRequest.

Change-Id: I30f2f5fc5d3383f942d0ad3dc01584c69c25ed25
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2612027
Commit-Queue: Daniel Rubery <drubery@chromium.org>
Commit-Queue: Xinghui Lu <xinghuilu@chromium.org>
Auto-Submit: Daniel Rubery <drubery@chromium.org>
Reviewed-by: Xinghui Lu <xinghuilu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840409}

components/safe_browsing/content/password_protection/password_protection_request.cc

@@ -540,7 +540,7 @@ void PasswordProtectionRequest::SendRequest() {
   url_loader_->DownloadToStringOfUnboundedSizeUntilCrashAndDie(
       password_protection_service_->url_loader_factory().get(),
       base::BindOnce(&PasswordProtectionRequest::OnURLLoaderComplete,
-                     base::Unretained(this)));
+                     GetWeakPtr()));
 }
 
 void PasswordProtectionRequest::StartTimeout() {