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..6dfeb9172fd3239f930575fe7f1f14bdc60af5f0 100644
--- a/base/files/file.cc
+++ b/base/files/file.cc
@@ -63,6 +63,15 @@ File::~File() {
   Close();
 }
 
+// static
+File File::CreateForAsyncHandle(PlatformFile platform_file) {
+  File file(platform_file);
+  // It would be nice if we could validate that |platform_file| was opened with
+  // FILE_FLAG_OVERLAPPED on Windows but this doesn't appear to be possible.
+  file.async_ = true;
+  return file.Pass();
+}
+
 File& File::operator=(RValue other) {
   if (this != other.object) {
     Close();