[PM] Add a comment to PerformanceManager::GetTaskRunner

Change-Id: I7848f077a6b67623c2d3d6eb48090f7447dfce5f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2230329Reviewed-by: Sigurður Ásgeirsson <siggi@chromium.org>
Commit-Queue: Sébastien Marchand <sebmarchand@chromium.org>
Cr-Commit-Position: refs/heads/master@{#775268}

components/performance_manager/public/performance_manager.h

@@ -136,6 +136,14 @@ class PerformanceManager {
     return static_cast<DerivedType*>(object);
   }
 
+  // Returns the performance manager graph task runner. This is safe to call
+  // from any thread at any time between the creation of the thread pool and its
+  // destruction.
+  //
+  // NOTE: Tasks posted to this sequence from any thread but the UI thread, or
+  // on the UI thread after IsAvailable() returns false, cannot safely access
+  // the graph, graphowned objects or other performance manager related objects.
+  // In practice it's preferable to use CallOnGraph() whenever possible.
   static scoped_refptr<base::SequencedTaskRunner> GetTaskRunner();
 
  protected: