[vm] Don't enable the profiler by default. Enable the profiler at startup with --observe, or later …

runtime/observatory/lib/src/elements/class_allocation_profile.dart

Index: runtime/observatory/lib/src/elements/class_allocation_profile.dart
diff --git a/runtime/observatory/lib/src/elements/class_allocation_profile.dart b/runtime/observatory/lib/src/elements/class_allocation_profile.dart
index 88987ae20ee329dda17748dd59d3c342e07a4db5..c678b85215ed96167f07a95b45d44631d771fba4 100644
--- a/runtime/observatory/lib/src/elements/class_allocation_profile.dart
+++ b/runtime/observatory/lib/src/elements/class_allocation_profile.dart
@@ -73,11 +73,11 @@ class ClassAllocationProfileElement extends HtmlElement implements Renderable {
       return;
     }
     final content = [
-      new SampleBufferControlElement(_progress, _progressStream,
+      new SampleBufferControlElement(isolate.vm, _progress, _progressStream,
           selectedTag: _tag, queue: _r.queue)
         ..onTagChange.listen((e) {
           _tag = e.element.selectedTag;
-          _request();
+          _request(forceFetch: true);
         })
     ];
     if (_progress.status == M.SampleProfileLoadingStatus.loaded) {