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

net/base/mock_file_stream.cc

Index: net/base/mock_file_stream.cc
diff --git a/net/base/mock_file_stream.cc b/net/base/mock_file_stream.cc
index 792b23b0344888ebb64c4dac10470c93bf7ebe05..014cace41c0e914325920ece1d897157221fe95b 100644
--- a/net/base/mock_file_stream.cc
+++ b/net/base/mock_file_stream.cc
@@ -33,13 +33,13 @@ MockFileStream::MockFileStream(
 MockFileStream::~MockFileStream() {
 }
 
-int MockFileStream::Seek(base::File::Whence whence, int64 offset,
+int MockFileStream::Seek(int64 offset,
                          const Int64CompletionCallback& callback) {
   Int64CompletionCallback wrapped_callback =
       base::Bind(&MockFileStream::DoCallback64,
                  weak_factory_.GetWeakPtr(), callback);
   if (forced_error_ == OK)
-    return FileStream::Seek(whence, offset, wrapped_callback);
+    return FileStream::Seek(offset, wrapped_callback);
   return ErrorCallback64(wrapped_callback);
 }