Add an "append flag" to base::PlatformFile.

base/platform_file.h

Index: base/platform_file.h
diff --git a/base/platform_file.h b/base/platform_file.h
index 6a6b9434fe58980624f1f2b39f7b1f9a896503e5..0010d4b6b2b618b6dbf3b940636d83b3db9ed380 100644
--- a/base/platform_file.h
+++ b/base/platform_file.h
@@ -49,6 +49,8 @@ enum PlatformFileFlags {
   PLATFORM_FILE_BACKUP_SEMANTICS = 1 << 17,  // Used on Windows only
 
   PLATFORM_FILE_EXECUTE = 1 << 18,           // Used on Windows only
+
+  PLATFORM_FILE_APPEND = 1 << 19,

[jar (doing other things), 2013/06/04 23:31:50]

nit: You *might* want to re-order these, so that WRITE and APPEND are next to
each other, now that you've made them exclusive.

Do you think there is a bunch of other code that would break?  Are there other
consumers or exporters of these flags???

 };
 
 // PLATFORM_FILE_ERROR_ACCESS_DENIED is returned when a call fails because of