Fix useless DCHECK in URLFetcherCore::StartURLRequest

The DCHECK on the pointer being non-null could be never hit,
as the pointer was dereferenced a couple of lines above.

BUG=none
TEST=existing tests

Review-Url: https://codereview.chromium.org/2732473003
Cr-Commit-Position: refs/heads/master@{#454471}
1 file changed