message: Explicitly disallow copy in PlatformThread
  Cr-Commit-Position: refs/heads/main@{#36494}
diff: @@ -55,6 +55,10 @@ class PlatformThread final {
  // removed.
  PlatformThread(PlatformThread&& rhs);
  + // Copies won't work since we'd have problems with joinable threads.
  + PlatformThread(const PlatformThread&) = delete;
  + PlatformThread& operator=(const PlatformThread&) = delete;
  +
  // Moves `rhs` into this, storing an empty state in `rhs`.
  // TODO(bugs.webrtc.org/12727) Look into if default and move support can be
  // removed.
  