Set request mode for beacon request with non-cors-safelisted blob

I removed the restriction on beacon with a blob with a non-CORS
safelisted content type at https://crrev.com/c/2011786 but forgot to
set the request mode. Fix that.

Bug: 1051368
Change-Id: Ibb843c2232af09390ca1155bd8a807c453c61683
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2050410Reviewed-by: Yoav Weiss <yoavweiss@chromium.org>
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#740611}

third_party/blink/renderer/core/loader/ping_loader.cc

@@ -39,6 +39,7 @@
 #include "third_party/blink/renderer/core/frame/local_frame_client.h"
 #include "third_party/blink/renderer/core/html/forms/form_data.h"
 #include "third_party/blink/renderer/core/typed_arrays/dom_array_buffer_view.h"
+#include "third_party/blink/renderer/platform/loader/cors/cors.h"
 #include "third_party/blink/renderer/platform/loader/fetch/fetch_context.h"
 #include "third_party/blink/renderer/platform/loader/fetch/fetch_initiator_type_names.h"
 #include "third_party/blink/renderer/platform/loader/fetch/fetch_utils.h"
@@ -110,8 +111,12 @@ class BeaconBlob final : public Beacon {
 
     request.SetHttpBody(std::move(entity_body));
 
-    if (!content_type_.IsEmpty())
+    if (!content_type_.IsEmpty()) {
+      if (!cors::IsCorsSafelistedContentType(content_type_)) {
+        request.SetMode(network::mojom::blink::RequestMode::kCors);
+      }
       request.SetHTTPContentType(content_type_);
+    }
   }
 
   const AtomicString GetContentType() const override { return content_type_; }

third_party/blink/web_tests/external/wpt/beacon/beacon-preflight-failure.sub.window.js

+// META: script=/common/utils.js
+// META: script=/common/get-host-info.sub.js
+
+promise_test(async (test) => {
+  const sid = token();
+  const tid = token();
+  const origin = get_host_info().REMOTE_ORIGIN;
+  const store =
+    `${origin}/beacon/resources/beacon.py?cmd=store&sid=${sid}&tid=${tid}&tidx=0`;
+  const monitor =
+    `/beacon/resources/beacon.py?cmd=stat&sid=${sid}&tidx_min=0&tidx_max=0`;
+
+  assert_true(navigator.sendBeacon(store, new Blob([], {type: 'x/y'})));
+
+  let actual;
+  for (let i = 0; i < 30; ++i) {
+    await new Promise(resolve => test.step_timeout(resolve, 10));
+
+    const response = await fetch(monitor);
+    const obj = await response.json();
+    if (obj.length > 0) {
+      actual = JSON.stringify(obj);
+      break;
+    }
+  }
+
+  const expected =
+    JSON.stringify([{id: tid, error: 'Preflight not expected.'}]);
+
+  assert_equals(actual, expected);
+});