appcache: remove spurious dcheck

This DCHECK happens if the data pipe that the appcache url loader is
writing the response body into for the renderer to read is closed.
This case is already handled (throwing a mojo error that the renderer
side can handle), and so don't DCHECK(false).

Bug: 1063255
Change-Id: Ic0d0af3e576d831251d878fb38db6438713bc652
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2510875
Commit-Queue: enne <enne@chromium.org>
Commit-Queue: Marijn Kruisselbrink <mek@chromium.org>
Auto-Submit: enne <enne@chromium.org>
Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822834}

content/browser/appcache/appcache_url_loader.cc

@@ -287,7 +287,6 @@ void AppCacheURLLoader::OnResponseBodyStreamReady(MojoResult result) {
   // TODO(ananta)
   // Add proper error handling here.
   if (result != MOJO_RESULT_OK) {
-    DCHECK(false);
     NotifyCompleted(net::ERR_FAILED);
     return;
   }