Gracefully handle nested eventSender.beginDragWithFiles() attempts.

components/test_runner/event_sender.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "components/test_runner/event_sender.h"
 
 #include <stddef.h>
 
 #include "base/logging.h"
 #include "base/macros.h"
@@ skipping 1827 matching lines @@
     SavedEvent saved_event;
     saved_event.type = SavedEvent::TYPE_LEAP_FORWARD;
     saved_event.milliseconds = milliseconds;
     mouse_event_queue_.push_back(saved_event);
   } else {
     DoLeapForward(milliseconds);
   }
 }
 
 void EventSender::BeginDragWithFiles(const std::vector<std::string>& files) {
+  if (!current_drag_data_.isNull()) {
+    // Nested dragging not supported, fuzzer code a likely culprit.
+    // Cancel the current drag operation and throw an error.
+    KeyDown("escape", 0, DOMKeyLocationStandard);
+    v8::Isolate* isolate = blink::mainThreadIsolate();
+    isolate->ThrowException(v8::Exception::Error(
+        gin::StringToV8(isolate,
+                        "Nested beginDragWithFiles() not supported.")));
+    return;
+  }
   current_drag_data_.initialize();
   WebVector<WebString> absolute_filenames(files.size());
   for (size_t i = 0; i < files.size(); ++i) {
     WebDragData::Item item;
     item.storageType = WebDragData::Item::StorageTypeFilename;
     item.filenameData = delegate_->GetAbsoluteWebStringFromUTF8Path(files[i]);
     current_drag_data_.addItem(item);
     absolute_filenames[i] = item.filenameData;
   }
   current_drag_data_.setFilesystemId(
@@ skipping 791 matching lines @@
   last_event_timestamp_ = event.timeStampSeconds;
 
   if (WebPagePopup* popup = view_->pagePopup()) {
     if (!WebInputEvent::isKeyboardEventType(event.type))
       return popup->handleInputEvent(event);
   }
   return view_->handleInputEvent(event);
 }
 
 }  // namespace test_runner