Eliminate spurious request cancelations

This fixes the situation where every request is canceled even after it
has completed successfully.  The cancels were not harming anything and
were dropped on the floor but the additional messages were useless.

BUG=642172

Review-Url: https://codereview.chromium.org/2303733004
Cr-Commit-Position: refs/heads/master@{#417565}

content/browser/loader/resource_dispatcher_host_impl.cc

@@ -1762,13 +1762,13 @@ void ResourceDispatcherHostImpl::OnCancelRequest(int request_id) {
     return;
 
   ResourceLoader* loader = GetLoader(child_id, request_id);
-  if (!loader) {
-    // We probably want to remove this warning eventually, but I wanted to be
-    // able to notice when this happens during initial development since it
-    // should be rare and may indicate a bug.
-    DVLOG(1) << "Canceling a request that wasn't found";
+
+  // It is possible that the request has been completed and removed from the
+  // loader queue but the client has not processed the request completed message
+  // before issuing a cancel. This happens frequently for beacons which are
+  // canceled in the response received handler.
+  if (!loader)
     return;
-  }
 
   loader->CancelRequest(true);
 }

content/child/resource_dispatcher.cc

@@ -488,8 +488,9 @@ void ResourceDispatcher::Cancel(int request_id) {
       should_dump = false;
     }
   }
-  // Cancel the request, and clean it up so the bridge will receive no more
-  // messages.
+  // Cancel the request if it didn't complete, and clean it up so the bridge
+  // will receive no more messages.
+  if (info.completion_time.is_null())
     message_sender_->Send(new ResourceHostMsg_CancelRequest(request_id));
   RemovePendingRequest(request_id);
 }

content/child/resource_dispatcher_unittest.cc

@@ -637,8 +637,8 @@ TEST_F(ResourceDispatcherTest, CancelDuringCallbackWithWrapperPeer) {
   NotifyRequestComplete(id, strlen(kTestPageContents));
 
   EXPECT_TRUE(peer_context.received_response);
-  // Request should have been cancelled.
-  ConsumeCancelRequest(id);
+  // Request should have been cancelled with no additional messages.
+  EXPECT_EQ(0u, queued_messages());
   EXPECT_TRUE(peer_context.cancelled);
 
   // Any future messages related to the request should be ignored.