[PDF Plugin] Increase read buffer size

pdf/document_loader.h

 // Copyright (c) 2010 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.
 
 #ifndef PDF_DOCUMENT_LOADER_H_
 #define PDF_DOCUMENT_LOADER_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
@@ skipping 90 matching lines @@
 
   Client* const client_;
   std::string url_;
   std::unique_ptr<URLLoaderWrapper> loader_;
 
   pp::CompletionCallbackFactory<DocumentLoader> loader_factory_;
 
   DataStream chunk_stream_;
   bool partial_loading_enabled_ = true;
   bool is_partial_loader_active_ = false;
-  char buffer_[DataStream::kChunkSize];
+
+  static constexpr uint32_t kReadBufferSize = 256 * 1024;
+  char buffer_[kReadBufferSize];
 
   Chunk chunk_;
   RangeSet pending_requests_;
   uint32_t count_of_bytes_received_ = 0;
 };
 
 }  // namespace chrome_pdf
 
 #endif  // PDF_DOCUMENT_LOADER_H_