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..b8fb512dab95472af942c2f5d3a0b33695b6f565 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.

[scottmg, 2015/06/15 22:06:54]

Actually, I think NtQueryObject will return the creation flags when retrieving
OBJECT_BASIC_INFORMATION, but I'm not sure how important it is to do that here.

[Reilly Grant (use Gerrit), 2015/06/15 22:36:36]

Microsoft also appears to be threatening to remove that function.

[scottmg, 2015/06/15 22:42:12]

Nah, it's just "internal"/"not documented", but it's never going anywhere. We
also use it quite a bit elsewhere fwiw:
https://code.google.com/p/chromium/codesearch#search/&q=ntqueryobject&sq=pack...
But I don't think the assertion is probably worth the complexity here.

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