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

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

Index: runtime/observatory/lib/src/elements/cpu_profile.dart
diff --git a/runtime/observatory/lib/src/elements/cpu_profile.dart b/runtime/observatory/lib/src/elements/cpu_profile.dart
index 68fba8acb83baf3b39db015e4a10e141f13ebe24..f5e5180fae62efe41dfc3e3ec9a929208fabab54 100644
--- a/runtime/observatory/lib/src/elements/cpu_profile.dart
+++ b/runtime/observatory/lib/src/elements/cpu_profile.dart
@@ -110,11 +110,11 @@ class CpuProfileElement 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;