clang-format: [JS] do not wrap after "readonly".

Summary:
Breaks after "readonly" trigger automatic semicolon insertion in field
declarations.

Reviewers: krasimir, djasper

Subscribers: klimek

Differential Revision: https://reviews.llvm.org/D35112

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@307394 91177308-0d34-0410-b5e6-96231b3b80d8