Check the result of DocumentThreadableLoader::create in mainThreadCreateLoader.

This CL is almost same as https://codereview.chromium.org/78393002 which is reverted in https://codereview.chromium.org/84883004/.
In this CL I reduced the file read request count in the test case to avoid another crash bug (http://crbug.com/295428).

BUG=

Review URL: https://codereview.chromium.org/84693006

git-svn-id: svn://svn.chromium.org/blink/trunk@162616 bbb929c8-8fbe-4397-9dbb-9b2b20218538