Add check to net:TestDelegate to check for response finishing early

If "RunUntilRedirect" or "RunUntilAuthRequired" is run and the request
unexpectedly finishes, TestDelegate will crash for trying to run the
"on_complete_" closure, which would be null, and crashes with a very
unhelpful message. A DCHECK and comment have been added that should make
The issue a little more clear.

Bug: 1000299
Change-Id: I537e8f2abc7754323bdaedcd7ee5d802ea674c9c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1782502Reviewed-by: David Benjamin <davidben@chromium.org>
Commit-Queue: Aaron Tagliaboschi <aarontag@chromium.org>
Cr-Commit-Position: refs/heads/master@{#693120}

net/url_request/url_request_test_util.cc

@@ -289,6 +289,12 @@ void TestDelegate::OnReadCompleted(URLRequest* request, int bytes_read) {
   // It doesn't make sense for the request to have IO pending at this point.
   DCHECK_NE(bytes_read, ERR_IO_PENDING);
 
+  // If you've reached this, you've either called "RunUntilComplete" or are
+  // using legacy "QuitCurrent*Deprecated". If this DCHECK fails, that probably
+  // means you've run "RunUntilRedirect" or "RunUntilAuthRequired" and haven't
+  // redirected/auth-challenged
+  DCHECK(on_complete_ || use_legacy_on_complete_);
+
   // If the request was cancelled in a redirect, it should not signal
   // OnReadCompleted. Note that |cancel_in_rs_| may be true due to
   // https://crbug.com/564848.