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 489216fbc24879faa5e048ad1730b2de310c1c9d..2f66b6e0314e70f67d972c6b0940ad4a7cd85e56 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 offset) {
-  int64 res = file_.Seek(whence, offset);
+  int64 res = file_.Seek(base::File::FROM_BEGIN, offset);
   if (res == -1)
     return IOResult::FromOSError(errno);