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..1bd680feea9aae97dc31105c09bc5b7d5b5095e0 100644
--- a/net/base/file_stream_context.h
+++ b/net/base/file_stream_context.h
@@ -92,9 +92,8 @@ 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);
 
@@ -149,7 +148,7 @@ 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();