Remove the capacity to cancel a load via nulling the ResourceRequest inside dispatchWillSendRequest().

Also, remove associated tests.

This capacity is only used in content_shell.

BUG=none

Review URL: https://chromiumcodereview.appspot.com/19741005

git-svn-id: svn://svn.chromium.org/blink/trunk@154779 bbb929c8-8fbe-4397-9dbb-9b2b20218538