Revert "Check the result of DocumentThreadableLoader::create in WorkerThreadableLoader::MainThreadBridge::mainThreadCreateLoader."

This reverts commit 6d7c391ae3e2c185ca2fa17a282e442225d9d551.
Original change: https://codereview.chromium.org/78393002

The added test has been crashing/timing out on Mac.

http://test-results.appspot.com/dashboards/flakiness_dashboard.html#tests=worker-read-blob-async-crash.html

TBR=horo@chromium.org

Review URL: https://codereview.chromium.org/84883004

git-svn-id: svn://svn.chromium.org/blink/trunk@162597 bbb929c8-8fbe-4397-9dbb-9b2b20218538