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

base/files/file.cc

Index: base/files/file.cc
diff --git a/base/files/file.cc b/base/files/file.cc
index 58f80c52322ec38125e1acbe3fda1120f64aa912..beee5ef10fee7eecf823cb82a0f4b9133584b5fd 100644
--- a/base/files/file.cc
+++ b/base/files/file.cc
@@ -63,6 +63,13 @@ File::~File() {
   Close();
 }
 
+// static
+File File::CreateForAsyncHandle(PlatformFile platform_file) {
+  File file(platform_file);
+  file.async_ = true;
+  return file;
+}
+
 File& File::operator=(RValue other) {
   if (this != other.object) {
     Close();