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

base/files/file.h

Index: base/files/file.h
diff --git a/base/files/file.h b/base/files/file.h
index b21b15972bce7c62be0dad659cff4e76af262ac2..d57d1d6bac73aba009fbd4038a52d6dff101ca0e 100644
--- a/base/files/file.h
+++ b/base/files/file.h
@@ -168,6 +168,9 @@ class BASE_EXPORT File {
   // Takes ownership of |platform_file|.
   explicit File(PlatformFile platform_file);
 
+  // Takes ownership of |platform_file| and sets |async|.
+  File(PlatformFile platform_file, bool async);

[Lei Zhang, 2015/05/25 00:29:58]

Calling this with |async| set to false is the same as the ctor above. Maybe do
"static File CreateForAsyncHandle(PlatformFile platform_file)" instead?

[Reilly Grant (use Gerrit), 2015/05/26 23:41:33]

Done.

+
   // Creates an object with a specific error_details code.
   explicit File(Error error_details);