[Merge] CaptivePortalDetector: Restore DCHECKs and add safety test.

This downgrades some CHECKS back to DCHECKs after the investigation
for crbug.com/1361443 has completed.

It also adds a safety check to prevent crashes if DetectCaptivePortal
is called incorrectly and a comment describing the correct usage.

BUG=1382965
TEST=No more production crashes in CaptivePortalDetector::DetectCaptivePortal

(cherry picked from commit 681f054)

Change-Id: I8c656b772411d11b68ff3ed742d6caf3abe7664a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4020985
Reviewed-by: Matt Menke <mmenke@chromium.org>
Commit-Queue: Steven Bennetts <stevenjb@chromium.org>
Cr-Original-Commit-Position: refs/heads/main@{#1070449}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4026426
Commit-Queue: Matt Menke <mmenke@chromium.org>
Auto-Submit: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/branch-heads/5414@{#35}
Cr-Branched-From: 4417ee5-refs/heads/main@{#1070088}

components/captive_portal/core/captive_portal_detector.cc

@@ -36,9 +36,10 @@ void CaptivePortalDetector::DetectCaptivePortal(
     const net::NetworkTrafficAnnotationTag& traffic_annotation) {
   DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
   DCHECK(!FetchingURL());
-  CHECK(detection_callback_.is_null());
-  CHECK(!detection_callback.is_null());
+  DCHECK(!detection_callback.is_null());
 
+  if (!detection_callback_.is_null())
+    LOG(ERROR) << "DetectCaptivePortal called while request is pending.";
   detection_callback_ = std::move(detection_callback);
 
   StartProbe(traffic_annotation, url);
@@ -83,7 +84,7 @@ void CaptivePortalDetector::OnSimpleLoaderComplete(
   DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
   CHECK_EQ(state_, State::kProbe);
   CHECK(FetchingURL());
-  CHECK(!detection_callback_.is_null());
+  DCHECK(!detection_callback_.is_null());
 
   int response_code = 0;
   net::HttpResponseHeaders* headers = nullptr;

components/captive_portal/core/captive_portal_detector.h

@@ -53,7 +53,10 @@ class CAPTIVE_PORTAL_EXPORT CaptivePortalDetector {
   ~CaptivePortalDetector();
 
   // Triggers a check for a captive portal. After completion, runs the
-  // |callback|.
+  // |callback|. Only one detection attempt is expected to be in progress.
+  // If called again before |callback| is invoked, Cancel() should be called
+  // first, otherwise the first request and callback will be implicitly
+  // cancelled. and an ERROR will be logged
   void DetectCaptivePortal(
       const GURL& url,
       DetectionCallback callback,