message: Add enable_shared_from_this to CompletionSession
diff: #include <mutex>
  #include <string>
  -struct CompletionSession {
  +struct CompletionSession : public std::enable_shared_from_this<CompletionSession> {
  Project::Entry file;
  WorkingFiles* working_files;
  clang::Index index;
  @@ -81,8 +81,8 @@ struct ClangCompleteManager {
  std::shared_ptr<CompletionSession> TryGetSession(const std::string& filename, bool create_if_needed);
  // TODO: make these configurable.
  - const int kMaxViewSessions = 3;
  - const int kMaxEditSessions = 10;
  + const int kMaxViewSessions = 1;
  + const int kMaxEditSessions = 1;
  // Global state.
  Config* config_;
  