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..cad62ac60c2623d60d682da5921a83697e3c5540 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 FLAG_OVERLAPPED on Windows but this doesn't appear to be possible.

[scottmg, 2015/06/15 21:42:45]

nit; FILE_FLAG_OVERLAPPED (I don't know of a direct way to check either)

[Reilly Grant (use Gerrit), 2015/06/15 22:01:12]

Done.

+  file.async_ = true;
+  return file;
+}
+
 File& File::operator=(RValue other) {
   if (this != other.object) {
     Close();