message: Remove forward declares
  This was missed in
  Cr-Commit-Position: refs/heads/main@{#38101}
diff: #include "rtc_base/experiments/field_trial_parser.h"
  #include "rtc_base/synchronization/mutex.h"
  #include "rtc_base/thread_annotations.h"
  +#include "system_wrappers/include/clock.h"
  namespace webrtc {
  -class Clock;
  -class TimestampExtrapolator;
  -
  class VCMTiming {
  public:
  static constexpr auto kDefaultRenderDelay = TimeDelta::Millis(10);
  