Fix SimpleURLLoader test fixture.

MockURLLoader::RunTest had a case statement that was accidetnally
falling through to the next case.

Bug: 177475, 746977
Change-Id: Ifce64d1afaaaff6b33ef8057517627e694389641
Reviewed-on: https://chromium-review.googlesource.com/891739Reviewed-by: Randy Smith <rdsmith@chromium.org>
Commit-Queue: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532933}

content/public/common/simple_url_loader_unittest.cc

@@ -1347,8 +1347,10 @@ class MockURLLoader : public network::mojom::URLLoader {
           status.error_code = net::OK;
           status.decoded_body_length = CountBytesToSend() - 1;
           client_->OnComplete(status);
+          break;
         }
         case TestLoaderEvent::kClientPipeClosed: {
+          DCHECK(client_);
           EXPECT_TRUE(binding_.is_bound());
           client_.reset();
           break;