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

net/base/file_stream_context.h

 // Copyright (c) 2012 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.
 
 // This file defines FileStream::Context class.
 // The general design of FileStream is as follows: file_stream.h defines
 // FileStream class which basically is just an "wrapper" not containing any
 // specific implementation details. It re-routes all its method calls to
 // the instance of FileStream::Context (FileStream holds a scoped_ptr to
 // FileStream::Context instance). Context was extracted into a different class
@@ skipping 74 matching lines @@
   // deferred if some asynchronous operation is now in progress or if file is
   // not closed yet.
   void Orphan();
 
   void Open(const base::FilePath& path,
             int open_flags,
             const CompletionCallback& callback);
 
   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);
 
  private:
   struct IOResult {
     IOResult();
     IOResult(int64 result, logging::SystemErrorCode os_error);
     static IOResult FromOSError(logging::SystemErrorCode os_error);
 
     int64 result;
@@ skipping 34 matching lines @@
   // network error code.
   void OnAsyncCompleted(const Int64CompletionCallback& callback,
                         const IOResult& result);
 
   ////////////////////////////////////////////////////////////////////////////
   // Platform-dependent methods implemented in
   // file_stream_context_{win,posix}.cc.
   ////////////////////////////////////////////////////////////////////////////
 
   // Adjusts the position from where the data is read.
-  IOResult SeekFileImpl(base::File::Whence whence, int64 offset);
+  IOResult SeekFileImpl(int64 offset);
 
   void OnFileOpened();
 
 #if defined(OS_WIN)
   void IOCompletionIsPending(const CompletionCallback& callback, IOBuffer* buf);
 
   // Implementation of MessageLoopForIO::IOHandler.
   void OnIOCompleted(base::MessageLoopForIO::IOContext* context,
                      DWORD bytes_read,
                      DWORD error) override;
@@ skipping 70 matching lines @@
   // Tracks the result of the IO completion operation. Set in OnIOComplete.
   int result_;
 #endif
 
   DISALLOW_COPY_AND_ASSIGN(Context);
 };
 
 }  // namespace net
 
 #endif  // NET_BASE_FILE_STREAM_CONTEXT_H_