Adding DumpWithoutCrashing if StartLoad happens in an MHTML.

MHTML documents should never trigger HTML requests.  Let's double-check
this with an explicit DumpWithoutCrashing added in this CL.

Bug: 1151438
Change-Id: I7a518308d85d58cb15deeec579df2f1c41e6abb6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2597028
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Auto-Submit: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#838787}

third_party/blink/renderer/platform/loader/fetch/resource_fetcher.cc

@@ -1984,6 +1984,15 @@ bool ResourceFetcher::StartLoad(Resource* resource,
 
   ResourceLoader* loader = nullptr;
 
+  if (archive_ && resource->Url().ProtocolIsInHTTPFamily()) {
+    // MHTML documents should not trigger HTTP requests.
+    //
+    // TODO(lukasza): https://crbug.com/1151438: Remove the ad-hoc debugging
+    // below, once the bug is understood.
+    NOTREACHED();
+    base::debug::DumpWithoutCrashing();
+  }
+
   {
     // Forbids JavaScript/revalidation until start()
     // to prevent unintended state transitions.