Use WeakPtr for MarkProxiesAsBad callback

This was causing a crash if the throttle was destroyed before the
callback was called.

Bug: 914436
Change-Id: Id52e41c2f42fe29116a2b877311cc1d9405b5116
Reviewed-on: https://chromium-review.googlesource.com/c/1374159Reviewed-by: rajendrant <rajendrant@chromium.org>
Commit-Queue: Clark DuVall <cduvall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616003}

components/data_reduction_proxy/content/common/data_reduction_proxy_url_loader_throttle.cc

@@ -146,7 +146,7 @@ void DataReductionProxyURLLoaderThrottle::MarkProxiesAsBad(
 
   auto callback = base::BindOnce(
       &DataReductionProxyURLLoaderThrottle::OnMarkProxiesAsBadComplete,
-      base::Unretained(this));
+      weak_factory_.GetWeakPtr());
 
   waiting_for_mark_proxies_ = true;
   manager_->MarkProxiesAsBad(bypass_duration, proxy_list, std::move(callback));

components/data_reduction_proxy/content/common/data_reduction_proxy_url_loader_throttle.h

@@ -78,6 +78,8 @@ class DataReductionProxyURLLoaderThrottle : public content::URLLoaderThrottle {
 
   // The final load flags used to complete the request.
   int final_load_flags_ = 0;
+
+  base::WeakPtrFactory<DataReductionProxyURLLoaderThrottle> weak_factory_{this};
 };
 
 }  // namespace data_reduction_proxy