Summary: I want to use TestLogHandler::getMessageValues with a const TestLogHandler reference. Mark getMessageValues as const so it can be called on a const TestLogHandler. getMessageValues doesn't mutate class state, so this should be safe.

Reviewed By: chadaustin

Differential Revision: D9815552

fbshipit-source-id: 275625d1a001fe536586229228eb2c9eee7e6330