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..819e8538c6c77abcaf654341c5aad98d7cca5891 100644
--- a/base/files/file.cc
+++ b/base/files/file.cc
@@ -44,6 +44,16 @@ File::File(PlatformFile platform_file)
 #endif
 }
 
+File::File(PlatformFile platform_file, bool async)
+    : file_(platform_file),
+      error_details_(FILE_OK),
+      created_(false),
+      async_(async) {
+#if defined(OS_POSIX)
+  DCHECK_GE(platform_file, -1);
+#endif
+}
+
 File::File(Error error_details)
     : error_details_(error_details),
       created_(false),