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 00b90e630f95d0d0ea85695451b4fbd28c36ec18..c1846c460d354a58b98d45b3d8694d5edf7f8651 100644
--- a/net/base/file_stream_context.h
+++ b/net/base/file_stream_context.h
@@ -75,7 +75,6 @@ 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_; }
 
   ////////////////////////////////////////////////////////////////////////////
@@ -93,12 +92,13 @@ class FileStream::Context {
 
   void Close(const CompletionCallback& callback);
 
-  void Seek(base::File::Whence whence,
-            int64_t offset,
-            const Int64CompletionCallback& callback);
+  // Seeks |offset| bytes from the start of the file.
+  void Seek(int64_t offset, const Int64CompletionCallback& callback);
 
   void Flush(const CompletionCallback& callback);
 
+  bool IsOpen() const;
+
  private:
   struct IOResult {
     IOResult();
@@ -150,7 +150,7 @@ class FileStream::Context {
   ////////////////////////////////////////////////////////////////////////////
 
   // Adjusts the position from where the data is read.
-  IOResult SeekFileImpl(base::File::Whence whence, int64_t offset);
+  IOResult SeekFileImpl(int64_t offset);
 
   void OnFileOpened();