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

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

Index: runtime/observatory/lib/src/elements/native_memory_profiler.dart
diff --git a/runtime/observatory/lib/src/elements/native_memory_profiler.dart b/runtime/observatory/lib/src/elements/native_memory_profiler.dart
index 4ce047e47ed886d3b6a24a0b0fb7bbe9cccc0735..f517447b4278359442997f97fd756a9cc731a76a 100644
--- a/runtime/observatory/lib/src/elements/native_memory_profiler.dart
+++ b/runtime/observatory/lib/src/elements/native_memory_profiler.dart
@@ -101,11 +101,11 @@ class NativeMemoryProfileElement extends HtmlElement implements Renderable {
       children = content;
       return;
     }
-    content.add(new SampleBufferControlElement(_progress, _progressStream,
+    content.add(new SampleBufferControlElement(_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) {
       CpuProfileVirtualTreeElement tree;