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

net/base/file_stream_context_posix.cc

Index: net/base/file_stream_context_posix.cc
diff --git a/net/base/file_stream_context_posix.cc b/net/base/file_stream_context_posix.cc
index dd62ebbe00b49faf816bf5777046e106ed922f48..fd6bf35b33a3df4f36ed00ba044639d48b411f50 100644
--- a/net/base/file_stream_context_posix.cc
+++ b/net/base/file_stream_context_posix.cc
@@ -79,9 +79,8 @@ int FileStream::Context::Write(IOBuffer* in_buf,
 }
 
 FileStream::Context::IOResult FileStream::Context::SeekFileImpl(
-    base::File::Whence whence,
     int64_t offset) {
-  int64_t res = file_.Seek(whence, offset);
+  int64_t res = file_.Seek(base::File::FROM_BEGIN, offset);
   if (res == -1)
     return IOResult::FromOSError(errno);