Only support seeking file streams from the beginning of the file.

net/base/file_stream_context.h

Index: net/base/file_stream_context.h
diff --git a/net/base/file_stream_context.h b/net/base/file_stream_context.h
index 74677fe8fb9edfdb86fd861121d451e6a5000e9f..a7f26f1b55722c47667bd06fdf4d0a06231b5dca 100644
--- a/net/base/file_stream_context.h
+++ b/net/base/file_stream_context.h
@@ -74,7 +74,6 @@ class FileStream::Context {
             int buf_len,
             const CompletionCallback& callback);
 
-  const base::File& file() const { return file_; }
   bool async_in_progress() const { return async_in_progress_; }
 
   ////////////////////////////////////////////////////////////////////////////
@@ -92,12 +91,13 @@ class FileStream::Context {
 
   void Close(const CompletionCallback& callback);
 
-  void Seek(base::File::Whence whence,
-            int64 offset,
-            const Int64CompletionCallback& callback);
+  // Seeks |offset| bytes from the start of the file.
+  void Seek(int64 offset, const Int64CompletionCallback& callback);
 
   void Flush(const CompletionCallback& callback);
 
+  bool IsOpen() const;
+
  private:
   struct IOResult {
     IOResult();
@@ -149,7 +149,7 @@ class FileStream::Context {
   ////////////////////////////////////////////////////////////////////////////
 
   // Adjusts the position from where the data is read.
-  IOResult SeekFileImpl(base::File::Whence whence, int64 offset);
+  IOResult SeekFileImpl(int64 offset);
 
   void OnFileOpened();