AU: SplitWriter class for parsing our full update files.

src/platform/update_engine/decompressing_file_writer.h

Index: src/platform/update_engine/decompressing_file_writer.h
diff --git a/src/platform/update_engine/decompressing_file_writer.h b/src/platform/update_engine/decompressing_file_writer.h
index 6f12e87b93e71c0d1d0c008dcc618bded508fe11..cd8e2946a870dac432c39406675ba179f1241d5e 100644
--- a/src/platform/update_engine/decompressing_file_writer.h
+++ b/src/platform/update_engine/decompressing_file_writer.h
@@ -36,7 +36,7 @@ class GzipDecompressingFileWriter : public FileWriter {
   // stream, after the entire stream has been written to this object,
   // the entire decompressed stream will have been written to the
   // underlying FileWriter.
-  virtual int Write(const void* bytes, size_t count);
+  virtual ssize_t Write(const void* bytes, size_t count);
 
   virtual int Close() {
     return next_->Close();