[clang-tidy] Set google-readability-namespace-comments.ShortNamespaceLines to 10

This value is used in cpplint, so we'd better be consistent.


git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@222431 91177308-0d34-0410-b5e6-96231b3b80d8