Make ResourceDispatcherHostImpl::DidReceiveResponse less repetitive

Initialise a |request| variable with the value of loader->request()
to avoid repeating it in 4 different places.

BUG=none
TEST=content_unittests

Review URL: https://codereview.chromium.org/1183723006

Cr-Commit-Position: refs/heads/master@{#335228}
1 file changed