Rm unimportant DCHECK in PrefetchProxy

Bug says this may fail on navigating to internal urls, but simply
clearing the ongoing prefetches again is harmless.

Bug: 1167997
Change-Id: I6ceb3525c61e9a593ecf8a298510df9c27d845ee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2636677
Commit-Queue: Robert Ogden <robertogden@chromium.org>
Commit-Queue: Ryan Sturm <ryansturm@chromium.org>
Auto-Submit: Robert Ogden <robertogden@chromium.org>
Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#844825}

chrome/browser/prefetch/prefetch_proxy/prefetch_proxy_tab_helper.cc

@@ -556,7 +556,8 @@ void PrefetchProxyTabHelper::DidFinishNavigation(
     return;
   }
 
-  DCHECK(!PrefetchingActive());
+  // Ensure there's no ongoing prefetches.
+  page_->url_loaders_.clear();
 
   GURL url = navigation_handle->GetURL();