Remove un-implemented functions from MultiSourceMemoryPressureMonitor.

Deletes the functions UpdatePressureLevel and NotifyListeners, which
were defined in the MultiSourceMemoryPressureMonitor header file but
never defined (as they were no longer used) in the source file.

Bug: 980965
Change-Id: I24d54177fb87de9e905caf4d157b7be328bcfd09
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1773846Reviewed-by: Sébastien Marchand <sebmarchand@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Commit-Queue: Ryan Powell <ryanpow@google.com>
Cr-Commit-Position: refs/heads/master@{#691642}

base/util/memory_pressure/multi_source_memory_pressure_monitor.h

@@ -48,9 +48,6 @@ class MultiSourceMemoryPressureMonitor
   void StartMetricsTimer();
   void StopMetricsTimer();
 
-  void UpdatePressureLevel(MemoryPressureLevel level);
-  void NotifyListeners();
-
  private:
   // Delegate implementation.
   void OnMemoryPressureLevelChanged(MemoryPressureLevel level) override;