Refactor PDF ChunkStream and DocumentLoader code.

pdf/chunk_stream.h

Index: pdf/chunk_stream.h
diff --git a/pdf/chunk_stream.h b/pdf/chunk_stream.h
index e669bb7727568ece9e3bb7ca22c795937981feea..762bc3925dd889707e22630df3551ba09c3cce44 100644
--- a/pdf/chunk_stream.h
+++ b/pdf/chunk_stream.h
@@ -41,9 +41,12 @@ class ChunkStream {
   size_t GetLastMissingByteInInterval(size_t offset) const;
 
  private:
+  std::map<size_t, size_t>::const_iterator GetStartChunk(size_t offset) const;
+
   std::vector<unsigned char> data_;
 
-  // Pair, first - begining of the chunk, second - size of the chunk.
+  // Key: offset of the chunk.
+  // Value: size of the chunk.
   std::map<size_t, size_t> chunks_;
 
   size_t stream_size_;