[Replaceable] attributes must be readonly

Web IDL spec: http://dev.w3.org/2006/webapi/WebIDL/#Replaceable

This change doesn't affect generated code, because the code generator
already assumes that [Replaceable] attributes are readonly.

R=abarth,eseidel

Review URL: https://chromiumcodereview.appspot.com/14589010

git-svn-id: svn://svn.chromium.org/blink/trunk@149756 bbb929c8-8fbe-4397-9dbb-9b2b20218538
2 files changed