IDL compiler: move [OverrideBuiltins] to interface

[OverrideBuiltins] goes on the interface, not on methods.

Simple fix, no changes to generated bindings.

Docs:
http://heycam.github.io/webidl/#OverrideBuiltins
https://sites.google.com/a/chromium.org/dev/blink/webidl/blink-idl-extended-attributes#TOC-OverrideBuiltins-i-

R=haraken

Review URL: https://codereview.chromium.org/150083002

git-svn-id: svn://svn.chromium.org/blink/trunk@166279 bbb929c8-8fbe-4397-9dbb-9b2b20218538