Added isolate + thread high watermark tracking to Observatory

runtime/vm/thread_registry.h

Index: runtime/vm/thread_registry.h
diff --git a/runtime/vm/thread_registry.h b/runtime/vm/thread_registry.h
index 881a4d0b03b517f98b8c8c07ecf2f32f40efad25..14220de79c544e719f3ebd694dbac6b24f647d7c 100644
--- a/runtime/vm/thread_registry.h
+++ b/runtime/vm/thread_registry.h
@@ -37,6 +37,9 @@ class ThreadRegistry {
   void PrintJSON(JSONStream* stream) const;
 #endif
 
+  // Calculates the sum of the max memory usage in bytes of each thread.
+  intptr_t ThreadHighWatermarksTotalLocked() const;
+
  private:
   Thread* active_list() const { return active_list_; }
   Monitor* threads_lock() const { return threads_lock_; }